I've always liked the fluff of spell preparation. Spells take a long time to cast, but if you prepare a spell beforehand, you can basically finish casting it quickly during combat. I could easily see a wizard that can't have a lot of spells prepared at a time, but who can prepare spells multiple times in a day.
This works well conceptually, but some spells would seriously impact the game if they could be cast more than once or twice per day.
A Possible Solution
The wizard has a number of spell slots equal to his level. Preparing a spell requires a number of slots equal to the spell's level and takes an amount of time noted in the spell's description.
Some spells, such as Burning Hands or Elemental Weapon take only a couple of rounds to prepare. Others, like Fireball or Tenser's Floating Disk take minutes, while powerful spells like Wish or Power Word Kill might take days or weeks.
