I think some great points have been made by Henry...(and kenobi65 too, of course!). I have been playing one particular wizard for several adventures and, over time, I think I've come up with a pretty good system for memorizing spells. Every level that I get to pick spells for my spellbook, I usually pick at least one big damage spell and one sneaky/fun spell. The beauty of spellcasting is that your useful both in and out of combat. I also apply that rule to preparing spells for the day. Usually I make sure I have at least one combative, and one non-combative spell per level memorized, even when we have made a specific plan...you never know when your going to need a non-combatative spell during heavy combat or when your going to need a big-ol-damage-doer when you've planed to sneak around (I have a penchant for rolling crap on a move silently check!).
Moreover, your wizard might want to be a bit more agressive when your party is making its plans. Make sure you say, "Ok, so I am memorizing THESE spells, so that we can...." a lot! The other players can't be held acountable for forgeting your character's weaknesses, so you sometimes have to remind them. Also, usually your spellcaster has a pretty decent intellegence, so use that to your advantage and take a more active role in the plan-making.
And finally, there is always going to be times when your plan goes ka-put, but thats no reason for you to give up on the spells you have memorized. Remember, you can get creative with your spells. Polymorph, for example, has great applications in combat as do a lot of other "sneaky" or "utility" spells.
Remember, Batman doesn't look at his batarangs and go...."Dang, these just have no use against the Joker, guess I'll sit this one out!"
Cheers!
T "Wizards are Like Batman" from Three Haligonians