Here are some of the things that I've done to promote 'balance' in the games that I've been running recently...
Here are the things I do that, while not intended to create "balance", have eliminated any angst about overpowered builds:
1) You always start at 1st level, with every PC. New joiners or replacements of the fallen aren't "builds" where you get to pick the magic items, etc., they are take over an NPC, who are universally weaker than the party members, or bring in an actual PC, leveled up from 1st level, developed in a different campaign (not necessarily with me as DM, and not necessarily created in this edition). Every PC needs to "earn his stripes" and be built organically, in actual play.
2) Default rules are core 3.5 books. Any additional rules need to be specifically approved -- player or DM nominated and DM approved. Our total rules outside the core are probably < 20 pages, and mostly feats from NBOF.
3) Play with people who don't care overwhelmingly much about D&D rules (storytellers, not build monkeys), mostly don't own a lot of non-core rules, are friends in real life and thus more amenable to compromise, and don't play all that often. If you get to play 2-3 times a year, a Human Fighter isn't boring. Not being jaded by continuous play makes the game a whole lot more interesting, and it's all we have time for.
4) Run a setting that assume Core assumptions -- the races in the PHB are dominant for PC's and almost all the "civilized" population, the classes in PHB + DMG Prestige Classes are nearly all that exist, the spells in the PHB are nearly all the spells known in the universe. And run adventures that are store-bought, and mostly assume pretty normal spells, classes, and monsters as opponents. Do that either by using adventures published early in 3e, or by removing most of the non-Core Materials, or non-conforming with the feel I'm looking for (e.g., no half dragons, ever).
5) 1st Edition feel, as I and everyone I know experienced AD&D in the good ol' days. Limited ability to buy or sell magic items. A world that exists NOT as a challenge to the specific party, but because it exists and has coherent history and ecology. Relatively "low powered" world, so being a Fireball-capably mage doesn't mean your some dude who works at the inn, it means you are a hero and probably known to the locals for the several adventures it took to reach that level of power. There will be stuff you can just stomp as a relatively high level PC (say 7th level or higher). There will be stuff you can't easily defeat and should avoid. This will be clear to everyone. Oh, and parties aren't with all PC's of uniform power. With 3.5 (or AD&D) XP rules, a 1st level character joining 3rd level PC's will catch up by 5th-6th level, if he or she survives. And that, believe it or not, is a fun challenge to us.
Right now in this adventure, my PC's are fighting half-orcs, humans of various classes (but mostly Warriors and Clerics for plot reasons), goblins, gargoyles, wererats, hell hounds, owlbears, a boss monster who's a Mind Flayer with character levels, and some extraplanar guys, plus possibly some giants later on. The only "weird" monsters than faced with a Cthulu-esque summoned monstrosity, and some "Tauric Hobgoblins" (hobgoblins with wings that they've encountered twice before). Discovering a Mind Flayer is the boss monster was a surprise, because we've never encountered one in this edition before -- most of us fought them in 1st Edition, but that was 20-30 years ago, and different rules.
Probably the most unusual aspect of my campaign is I sometimes have set piece battles, with scores of combatants on each side and the PC's commanding. It's actually pretty interesting . . . not easy for PC's to nova and specialize if they might be a multi-company scale battle that ends up strung out over hours. Makes fighter-types much more "balanced" when paired with "nova and then I must rest" casters. Riding and archer turn out to be useful.
