Whenever I have fireball or a similar AoE effect prepared I usually end up not using it all day. The reasons are that it is hard to use without harming allies or "wasting" the AoE effect on single targets.
I assume it is probably intended that Fireball is hard to use, since it is so powerful. But how do I use it as effectively as possible, in particular without harming my allies?
Best Answer
Go First
Do you have the Improved Initiative feat? If you take the first move in the combat, you can throw your fireball before your allies get mixed up with your enemies.
Use Metamagic
Feats such as Sculpt Spell will let you reshape an area spell, giving you more control over what it hits. The "four ten-foot cubes" option is particularly flexible.
In one game, one of my players was a warmage who threw sculpted fireballs all the time, and it was very effective.
Talk To Your Allies
Make sure your allies understand when you've got a fireball ready. After the monsters move, say something like: "guys, I'm thinking I might throw a fireball at those four monsters, please don't move into that area". If someone forgets, give them a polite reminder: "I'm planning to throw a fireball in that area, are you sure you want to move there?"
I once played a 4e wizard who relied heavily on area attacks. The party got used to it -- they'd be moving and I'd say "uh, maybe not that square?" and they'd go somewhere else instead.
I don't have a ton of experience of high-level play, but it may be worth mentioning: many people believe that direct-damage spells such as fireball are not the best use of a wizard's time (in 3.5e and Pathfinder). Monster hp and defenses scale up much faster than fireball damage, so your direct-damage spells will eventually look really inferior compared to other classes of spells. The treantmonk wizard guide has more on this philosophy. (The linked guide is for Pathfinder but websearch can find you similar guides for 3.5e.)