In addition to guidelines for effective development, I also offer some additional advice with specific reference to planning for the epic levels, the 21st to 30th levels. If you plan to play either or both of the Mask of the Betrayer and the Storm of Zehir expansions, then it helps to plan the development of an epic build from the moment you first create your characters. In other words, even your development from levels 1 to 20 should be done with foresight of how the character will further develop during levels 21 to 30. Now for some specific guidelines, along with specific examples along the way.
Epic Feats as your Linchpins
Some feats will just take your character's performance to a whole other level, sometimes to the point that they're the very reason for designing a build a certain way. I'll describe some examples.
Combat Insight - Allows a character to substitute the Intelligence bonus for the Strength bonus, if the former is higher, when it comes to bonus damage for a melee strike. It is this feat that underlies a Swashbuckler / Duelist build that can have Intelligence enhance bonus damage, and armor class. I describe this build in the walkthrough for Storm of Zehir. Likewise the feat is a foundation for Valeria's build, allowing Intelligence to increase her Wizard spells, the damage she inflicts with her melee attacks, and her skill point distribution.
Perfect Two-Weapon Fighting - For any character that emphasizes melee combat while dual-wielding two weapons, this is your target feat. It allows you to score several attacks with each weapon, and with no attack penalty.
Hymn of Requiem - The target feat for any Bard character. It is a passive inspiration that simultaneously inflicts sonic damage on all monsters in the radius of effect and heals all party members in the radius of effect. Its power increases with a Bard character's rank in the Perform skill.
Chorus of Heroism - Provides +4 to both saving throws and dodge bonus to armor class for all party members. Using the Chorus in conjunction with the Hymn of Requiem and the Curse Song can turn almost any combat into a lopsided affair.
Epic Precision - One thing to keep in mind is that MANY enemies in Mask of the Betrayer will be immune to critical hits and sneak attacks. If you want to play a Rogue-based character in Mask of the Betrayer, this should definitely a target feat. It allows a Rogue character to inflict half sneak attack damage on a target that would otherwise be immune to sneak attacks.
Epic Rage and Thundering Rage - Epic Rage will increase the Strength and Constitution bonuses of a Barbarian character to 10 each during a Barbarian Rage. Thundering Rage allows a Barbarian to inflict extra sonic damage on a critical hit while raging. Note again my comment that almost everything in Mask of the Betrayer is immune to critical hits. Thundering Rage may thus be a more suitable target during a Storm of Zehir playthrough than a Mask of the Betrayer playthrough.
Eldritch Master and Epic Eldritch Blast - Eldritch Master is a key target feat for an epic Warlock, as it increases Eldritch Blast damage by 50% and the attack bonus of the Eldritch Blast by +2. Epic Eldritch Blast can be taken multiple times to increase blast damage by +1d6 every instance. Eldritch Master coupled with multiple instances of Epic Eldritch Blast can turn the epic Warlock into a veritable cannon of destruction that can fire off highly damaging ammunition with no limit.
Expose Weakness - I'll explain in more detail how exactly this feat works when I describe how I develop Valeria. The short of it is that the feat allows your first attack to hit automatically (even through concealment), and has a chance of inflicting armor class penalties and Dexterity-based damage for five rounds thereafter. It can be combined with Improved Combat Expertise in a distinctive fighting style that's based on maximizing armor class, and always guaranteeing at least one successful hit each round that has a chance of inflicting extra damage and armor class penalties. As Combat Expertise is also a prerequisite for Combat Insight, it is entirely feasible for such a character build to include that feat as well. This is precisely part of the foundation for my character build for Valeria.
You will have to keep in mind that these epic feats will naturally have prerequisites, some of them quite hefty. Your character build, all the way from level 1 to level 30, must at least partially account for the acquisition of the prerequisites for the epic feats that you will want.
Lastly, some epic feats are not all that. Some really aren't worth going out of your way for. For example, Planar Turning sounds pretty cool. But outsiders are pretty rare in both Storm of Zehir and even Mask of the Betrayer. Add to that its hefty prerequisites, it's really not worth it. As another example, the Dragon Wild shape sounds really cool. But it is my considered opinion that the Elder Elemental Wild Shape is the best Wild Shape option for even epic level Druids. An Elder Elemental Druid can be pre-buffed with all of Greater Magic Weapon, Flaming Weapon, Premonition, and Barkskin. Such a Druid can also augment combat prowess even further with a Storm Avatar spell. Furthermore, the Elder Elemental form enjoys all of the elemental immunities, like immunity to paralysis, mind-affecting spells, and critical hits. No need to spend an epic feat on something that, in my honest opinion, doesn't stack up as well.
Epic Level Spells
All epic spells will bypass spell resistance. Here I'll offer a brief opinion on each one.
Damnation - Being a single-target death spell, really only a Cleric should take it.
Epic Gate - The ultimate meat shield. Summons one Balor that has plenty of hit points and a vorpal sword. Will summon another one immediately after the first one dies. Definitely recommended for at least one character in the party.
Entropic Husk - Too much of a wild card in my opinion, as the resulting creature is as likely to target the party as the other monsters.
Hellball - The ultimate damage spell and a target for any epic Sorcerer, Wizard, Warlock, or Stormlord. It not only bypasses spell resistance, but Evasion and Improved Evasion as well.
Mass Fowl - Will turn many of the monsters into harmless chickens. The downside to using this in Mask of the Betrayer to the point that it shouldn't be selected, in my honest opinion, is that you won't be able to collect any crafting essences from the poultry afterwards. Don't bother.
Vampiric Feast - It's like Wail of the Banshee on steroids. Will instantly kill any monster that fails its saving throw without harming party members, and will also replenish the character's hit points. It bypasses both spell resistance and Death Ward effects. It's entirely possible, even recommended, that an offensive spellcaster select both Hellball and Vampiric Feast during level ups. The more cards in your deck the better, right?
One Approach: 30 Levels in a Base Class
One idea is to take all 30 of your levels in one of the base classes of the D&D 3.5e rules system, no levels in any prestige class, no levels in any other class besides your one base class. There are definite advantages to this.
All epic characters get bonus feats at levels 21, 23, 25, 27 and 29. A character who devotes him or herself exclusively to a base class will get even more bonus feats in addition to the standard ones. If you want to base an epic build on this basis, the question you need to ask yourself is whether the bonus feats will outweigh the benefits of multi-classing into other classes, prestige classes included. The answer is that it may, depending on the build.
For example, a 30th level Sorcerer or Wizard may be able to rack up on the metamagic feats, the item crafting feats, Greater Charisma or Greater Intelligence, and the epic spells to boot. 30th level spellcasters gain an additional benefit as well, a +3 bonus to the Difficulty Class for saving throws against their spells. A 30th level Monk can continue to increase spell resistance, damage while fighting bare-handed, and armor class. A 30th level Bard enjoys increasingly powerful inspirations.
Sometimes a build can be based on compromising with this approach a little. Maybe it's worthwhile to take one or two levels in another class in order to gain a key benefit, while still leaving plenty of room for increased bonus feats and epic-level power for a base class.
For example, maybe a Rogue build only needs to take a single class of Shadowdancer. The key benefit of the 1st level of the Shadowdancer is the Hide in Plain Sight ability. Hide in Plain Sight, in combination with high Hide and Move Silently skills, allows a character to go into stealth mode even when in plain view of the monsters. The way to exploit this ability is to Hide in Plain Sight, wait until the 6 second cooldown period has elapsed, sneak attack a monster, and then use Hide in Plain Sight again. The ability can be used in a constant cycle of hiding and sneak attacking in the middle of combat whereby the Rogue is never in any real danger. Taking the other 29 levels in Rogue exclusively allows the character to increase his sneak attack damage, and rack up on feats like Epic Precision and multiple instances of Improved Sneak Attack.
A variation on the Hide in Plain Sight theme is the build I use for Finch during Storm of Zehir. Finch will rack up 28 Bard levels, thereby increasing the power of his songs and inspirations, and gaining access to abilities like Hymn of Requiem. He cherry picks one level of Warlock in order to gain access to the Dark One's Luck invocation, which will increase for 24 hours his saving throws by a bonus equivalent to his Charisma ability modifier. He also picks up one level of Shadowdancer. The idea is that he can plant himself in the thick of combat, unleash his inspirations and/or Curse Song and/or the Hymn of Requiem, and then he can use Hide in Plain Sight. That way, everybody, friends and foes alike, are within the radius of his powers, and yet he remains unnoticed while in stealth mode.
Another idea could be combining 3 levels of Arcane Scholar of Candlekeep with 27 levels of Sorcerer. I would say that getting the Improved Empower Feat from the 3 levels of Arcane Scholar is the only reason to get any levels in the prestige class, and there's no real reason to go beyond that. 27 levels in Sorcerer still allows you to rack up plenty of magic-enhancing feats, especially during levels 21 to 27, making for a very powerful damage caster build.
Another damage caster build is the Hellfire Warlock build that I use for Vandala during Storm of Zehir. She takes only one level in Cleric in order to score some bonus feats through the cleric's armor and shield proficiencies, as well as through certain domain choices. She takes three levels in Hellfire Warlock in order to increase her damage by 6d6. The rest are all Warlock levels, so that she can rack up on feats like Eldritch Master and multiple instances of Epic Eldritch Blast.
Another common build is 29 levels of Druid and a single level of Monk. The idea is that taking only one level of Monk allows the Druid to add the Wisdom bonus to armor class to his Wild Shape as well. Wisdom thus becomes the linchpin attribute, enhancing Druid spellcasting and the fighting capabilities of the Wild Shape form.
Another Druid-heavy build is the one that I used for Umoja during Storm of Zehir. Umoja pumps up his Wisdom as much as he can. He takes two levels in Ranger in order to be able to use a Longbow with the Rapid Shot feat. He also takes the Zen Archery feat. Wisdom therefore increases both his power as a Druid and his ability to hit his targets with his bow. He also has the best possible animal companion in the game, a dinosaur named Yushai. Umoja therefore also prioritizes maximizing the power of Yushai with feats like Natural Bond and Epic Animal Companion.
Another Approach: 20 Levels as a Spellcaster and 10 Levels in Other Classes
One fact to keep in mind is that for spellcaster classes, arcane or divine, their spell progression tables only go up to 20th level. You do not get anymore spells beyond what you've obtained, and beyond what your casting attribute bonus provides (e.g. bonus spells for Druids with a high Wisdom score).
The benefit to taking levels 21 to 30 in a spellcaster base class is to gain extra bonus feats, and bonuses to the DC for your spells. There is then the question of whether it's worthwhile to combine 20 levels in a spellcaster class and 10 levels in other classes (maybe a prestige class) that will give the character a greater range of abilities. Will that greater range of abilities outweigh losing out on the bonus feats and the increased spell DC. Some of those 20 spellcaster class levels can be obtained from prestige classes that themselves raise spellcaster level, like an Arcane Trickster. Maybe those 10 non-spellcaster levels can be derived from prestige classes as well. Keep in mind that a character can obtain levels in a total of four different classes, base or prestige.
This concept is the very foundation of my Arcane Trickster / Rogue / Swashbuckler build for Valeria. For her level-ups from 1 to 20 during the OC, see this link. For her development during the epic levels of MotB, see this link. There are other possibilities as well. Combining Sacred Fist and Cleric and Monk levels together is entirely sensible.
It is again possible to compromise, meaning not adhering strictly to a 20 / 10 distribution, and still come up with effective builds. For example, it is possible to combine together 2 Paladin levels with 8 Sorcerer levels, 10 Eldritch Knight levels, and 10 Red Dragon Disciple levels. With this build, Charisma will enhance both saving throws and Sorcerer spells. A Sorcerer spellcaster level of 17 (9 of those coming from taking the Eldritch Knight levels) will still give you significant spell power, including a 9th level spell of your choice. Combining Eldritch Knight levels with the armor class and attribute bonuses provided by the Red Dragon Disciple levels means considerable melee capability. Picture it, a character that can swing away for massive damage with a two-handed sword while protected by Premonition and Illusions, occasionally spitting out offensive spells for good measure.
If you want to do things this way, you need to keep in mind that you can only select an epic spell feat when you also take a level in the appropriate spellcaster class during that level up. For example, if you want a Wizard / Fighter build to take Hellball at the 25th level, you must take a level in Wizard when leveling up to the 25th level in order for Hellball to be available during feat selection. Taking a Fighter level when leveling up to 25th will mean that Hellball will not be available during level up.
What that means is that you should probably not take all of your spellcaster class levels during levels 1 to 20. Leave some of your spellcaster level ups for levels 21 to 30. For example, with Valeria I save five of her Wizard level ups for levels 21, 23, 25, 27, and 29, so that she has the option of selecting epic spells during those level ups.
Balancing between Harmony and More Abilities
The whole point of multiclassing is to add new abilities that would otherwise be inaccessible if you had just stuck with the same class the whole way through, with a view towards improving your character's efficacy and power.
What I mean here is that you must maintain a certain balance. Sure, it's nice to have more and more abilities. But it also helps if they complement each other, or have a certain harmony with each other. Accruing more abilities can be pointless, even counter-productive, if they're so disparate that they don't really complement or augment each other. That is especially the case when the abilities call upon different attributes (e.g. one relies on Strength while the other relies on Intelligence). I'll give a few examples of possible builds that I think strike a good balance.
The damage dealer build that I used for Logan during Storm of Zehir combines levels in Fighter, Divine Champion, Weapon Master and Frenzied Berserker. The point of Weaponmaster levels is to crank out highly damaging critical hits on a frequent basis. A two-handed weapon with a high critical hit range, like a Falchion, is what I have in mind. The Enhanced Power Attack ability of the Frenzied Berserker, which works best with two-handed weapons, will increase even more the critical hit damage inflicted by the build. Supreme Cleave from the Frenzied Berserker will mean two follow through hits on the next target after striking a killing blow. It is conceivable that Great Cleave and Supreme Cleave together, combined with highly damaging criticals from a two-handed weapon further augmented by Enhanced Power Attack, can result in the build killing several monsters outright in a single round. It goes without saying that all of the abilities of this build rely on Strength. Several abilities from different classes harmonized together, a build where the sum is greater than its individual parts.
The build I used for Gallard during Storm of Zehir more or less ties everything to Dexterity. The idea is a character that can provide basic thieving skills, and be incredibly difficult to hit due to a combination of very high armor class and a very high Parry skill rank. Duelist levels provide an immense bonus to both armor class and skill parry rank. Swashbuckler levels provide dodge bonuses to armor class, and bonus feats that are prerequisites for the Duelist prestige class. Divine Champion levels provide bonus feats that are integral to the build. Rogue levels allow the early accumulation of thieving skills, as well as the Evasion ability. Pumping up Dexterity enhances performance in every aspect of the build.
Another way to pursue that harmony is that some prestige classes will simultaneously enhance the abilities of more than one base class. For example, the Sacred Fist prestige class allows for a simultaneous improvement in both Monk fighting ability and Cleric spellcasting ability. Harmony is further enhanced by the fact that Wisdom simultaneously enhances abilities from classes as well.
As another example, the build I use for Janette during Storm of Zehir combines Favored Soul with Stormlord and Divine Champion levels. Favored Souls do not have access to abilities like Divine Might and Divine Shield, since they would need Turn Undead which is normally a Cleric class feature. All she'll need is a few Paladin levels sprinkled in, and voila she can access Divine Might and Divine Shield to increase her combat abilities. Charisma will augment both Favored Soul spellcasting and the use of Divine Might and Divine Shield, so there remains a definite harmony here. *Note* It is likewise very possible to design an evil counterpart, whereby 3 levels in Blackguard instead of Paladin give access to Turn Undead and in turn open access to Divine Might and Divine Shield. Be mindful that you'll need an evil alignment, 5 ranks in the Hide Skill, a base attack bonus of +6, and the Cleave and Great Cleave feats, in order to gain access to the Blackguard prestige class.
A last example of a build that I'll describe is one that combines the concept of harmonizing different abilities and also maximizing levels in a base class in order to attain bonus epic feats. It is my Cleric / Blackguard build that I use for my evil playthrough of the OC and MotB. It uses 26 levels in Cleric, three levels in Blackguard, and one level in Warpriest. The Cleric's domains are Air and Water, to gain access to the Chain Lightning and Ice Storm spells. It is a Damage Caster Cleric build, so I select Spellcasting Prodigy as one of my 1st level feats. Three levels in Blackguard provides the Aura of Despair ability, which imposes a -2 saving throw penalty on all enemies within a 10' radius. One level in Warpriest provides the War Glory aura, which imposes a -1 saving throw penalty on all enemies within a 10' radius. The penalties from both auras will stack. I can also use the Permanency feat to cast a Prayer spell that will last 24 hours, which provides another -1 saving throw penalty that will also stack. In the right conditions, that amounts to a -5 saving throw penalty for the monsters. The idea is a heavily armored and heavily buffed Cleric that can step right into the thick of things, and let loose with spells like Empowered Chain Lightning and Storm of Vengeance. The saving throw penalties will also benefit companion casters like Qara and Safiya. It goes without saying that I'll maximize Wisdom as much as possible. For his level-ups from 1 to 20 during the OC, see this link. For his development during the epic levels of MotB, see this link.