Mark Chance said:
There is no such thing as an invincible character.
Except Elminster.
interwyrm, name your character Elminster. Write the words "plot device" on your character sheet, and no DM will be able to touch you.
And now seriously...
You have to decide what you want to be invincible from. There's no way to defend against every eventuality, but you can become darned good at taking down (or at least neutralizing) a fair swath of enemies.
My Sorc/Iot7FV was a pretty darned good anti-mage. A Ring of Counterspells protected me from targetted
Dispel Magics. Contingent
Protection from Evil protected me from most Enchantment spells.
Fly, Swift kept me away from the melee types. And Wardings kept the pesky arrows at bay.
True, I'd run out of stuff eventually, but that would keep me very safe for about two encounters a day. And I'd be fairly free to dish out hurt while I was at it.
Funny thing was, I wasn't so hot at killing the enemy melee types because most of my spells were of the protective variety. Sure, they'd be inconvienienced by
Walls of Ice, but they'd still have to be killed later. So I realized that despite my design, I needed those meat shields because I couldn't take care of enemy thugs very easily.
The point being, with your wizard, try make the party invincible even if only for a short while. Keep the enemy spellcasters at bay, divide the enemy footsoldiers, and allow your team to do what it does best: the actual killing. Be a part of that team, and enjoy the role of keeping their buns out of the oven most of the time.
And while you're at it, try to always make it seem like one of your other party members is a more dangerous target than you.