So it's well-known (by me) that 3.5e's biggest flaw is balance between classes. (This flaw is slightly lessened but not really entirely fixed by Pathfinder.) Monks are crap, druids are boss. It is known.
In my opinion, the best way to fix this is to go well beyond standard wealth by level guidelines.
You can just spam extra gold and it'll ameliorate the problem, because a fighter is more improved by doubling his WBL than a druid is.
But there's an even better way, and that's to drop extra loot tailored to the party. Specifically, loot that the underperforming characters can use and characters performing adequately can't.
Is the party's monk underperforming? (Yes of course he is, don't be silly.) Give the next encounter a Monk's Belt and a Necklace of Natural Attacks.
Warlock? Chasuble of Fell Power (Magic Item Compendium). MIC has a lot of these items, tailored to specific classes in this way. Vest that gives the rogue more sneak attack, boots that give the scout more skirmish, etc.
Got a fighter that's getting outperformed by a warblade? Give a Holy/Unholy/Anarchic/Axiomatic weapon opposed to the warblade's alignment. Or the Crown of White Ravens and its brethren from Tome of Battle (this is useful to a warblade, so you'll need to encourage the warblade to give it up to the fighter), which serves as an introduction to ToB and might encourage the fighter to multiclass to an initiator class (which, unlike multiclassing into a spellcaster and being forever behind, is a favorable choice, because your initiator level is half your non-initiator-class levels plus your level in initiator classes, and which maneuvers/stances you can pick depends only on your initiator level, so a fighter 4/warblade 1 has an IL of 3 instead of 1 and can pick 2nd-level maneuvers right off the bat).
Even a wizard or an archivist can underperform through being played by an unsavvy player, so you can drop scrolls of better spells for them to copy into their spellbooks to subtly encourage their use. E.g., archivist wasting all his time healing? Drop him some divine scrolls of entangle and hold person and stuff. Alternately, and this works for pretty much any caster, drop a wand of something useful that's on the underperforming caster's class list.