I'm going to go out on a limb here and say that I think the rapid advancement of characters and then differentiating between levels 1 - 20 and above is a fundamental flaw in the design of 3E. In my opinion, epic becomes too powerful, and if anything, epic should slow the progression of characters way down. Characters should not be able to increase in power to the point where they become mortals with the power of demigods. This is one of the things that the Babylon 5 RPG does right - it keeps the power down to believable levels (even if I do think they lowball the hit points!).
That said, this is the system within which we must work, so differentiating between levels 20+ becomes an unavoidable evil. Listed below is how I would handle advancement beyond level 20. Keep in mind that this is not the advancement method listed in the 3.5 DMG, nor is it intended to give you a great deal of incentive to continue to advance your character. What it does is simply make it so that an "epic" character is not explonentially more powerful than a standard high level character, which keeps it playable within the confines of a game designed for levels 1 - 20.
Hit Points continue to increase by class (in my game, each class gets 1 more hit point per level than what is listed in the book, but would drop down to the increase listed in the book by class after level 20).
Skill Points increase by your Intelligence modifier or 1 (whichever is greater) per level.
Base Attack Bonus continues to increase, but at a much slower rate.
Characters with a good attack bonus (increases increase at a rate of 1:1 on levels 1 - 20) now increase at a rate of 1:5. Characters now gain +1 to attack at levels 25, 30, 35, 40, 45, 50....
Characters with a moderate attack bonus (increases at a rate of 3:4) now increase at a rate of 1:7. Characters now gain +1 to attack at levels 27, 34, 41, 49, 56, 63....
Characters with a poor attack bonus (increases at a rate of 1:2) now increase at a rate of 1:10. Characters now gain +1 to attack at levels 30, 40, 50, 60, 70....
No additional iterative attacks are gained after level 20.
Saves also increase at a slower rate for all characters. Your character's good save continues to increase, but at a rate of 1 point for every 5 levels. Your character's poor saves increase at a rate of 1 point for every 10 levels.
Class abilities, feats, and powers do not increase beyond level 20. Because characters no longer gain class abilities or feats, they are considered to be advancing in a core class they had previously taken and bases his advancement (as outlined above) on that class.
Like I said, this isn't a progression that greatly rewards characters for continuing to advance, but it does allow you to continue your game beyond level 20. A 100th level character is not 5 times more powerful than a 20th level character.