I'm a fan of "Vancian" casting. In my rationalization for it, the wizard creates magical "constructs" that exist only in a metamagical plane. The wizard carries these around with him and essentially throws them at the enemy like a grenade. If the wizard has a focus object, that object tethers the constructs. The casting ritual is the way to bring these constructs forth from the void and into reality. Metamagic Feats (as in 3e) involve the caster deriving new "designs" for these constructs, scrolls bind the constructs to paper with special inks, etc. Cantrips are spells so simple to construct that you can make them and release them in almost a single gesture (or word). Spell limits represent the wizard's ability to "multitask" and keep track of these constructs. When you sleep, your spells can "wander off", so it's best to dismantle them. If you go unconscious, you can probably keep your spells; I have the wizard make concentration checks for each spell left.
Sorcerers in this model can perceive the metamagic realm, and therefore can make whatever constructs they understand. There is still a limit on their stamina, though I'm coming around to the idea that it's more of a personally set limit, and the sorcerer realizes that to push beyond that limit most likely means losing control.
That's all fine and dandy for 3e or Pathfinder, but what about Old School D&D versions, like AD&D or my current brainworm, Swords and Wizardry?
S&W is a relatively faithful clone of the Original D&D (OD&D), which I never played. I started with Basic and then Advanced, which were a bit more detailed. S&W, like many old school games, doesn't really lay out a lot of rules. For example, though it says a Wizard can prepare X spells "per day", it does not define how long it takes to prepare them, how long you need to rest, or what happens if you go unconscious. Given the brutality of old school games, I have little doubt that an unconscious wizard should lose their spells, but that's a DM's judgement call.
So, since it's the DM's call, here are some ideas I'm thinking about for spell preparation in S&W:
First, it takes (Spell Level) divided by (Caster Level) hours to prepare each spell, plus one turn to set up/take down your preparation materials.
Second, each hour of rest will reset one spell level of slots. One hour resets your 1st level slots, 2 hours resets 1st and 2nd level, etc.
In other words, a 1st level wizard can re-prepare his spell after a 1 hour lunch, but it will take him an extra hour to do so. A second level wizard still only takes 1 hour to reset his 1st level slots, and can prepare twice as many spells per hour.
The upshot of this is that low level wizards can often have two "15 minute work days", and high level wizards may not bother preparing low level spells, since if the need arises, they can prepare such spells in a turn or two. A smart high level wizard will leave a slot or two empty so if, for example, the party really needs that Knock spell, he can spend some time preparing it.
Of course if the wizard is high level enough, he will prepare scrolls for these contingencies.