Please no Vancian system. I just can't understand "forgetting" a spell and having to "study" it again so you can cast it.
If you want there to be some artificial preparation to force magic users to guess what they'll need, the perhaps divide up magic into different elemental types, foci, or whatever division you want to include and force MUs to choose how much of what kind of mana/spell points they want to cast. Instead of 50 mana, make that 30 exothermic mana, 10 life mana, and 10 dark mana to draw upon.
Or do something like you can have X number of "fresh in your mind" spells with a base 100% chance of casting success and the rest with lower base chances of success based on spell difficulty and how much experience you have casting it. (So that eventually simple spells you've used thousands of times are as effortless as "fresh in your mind" spells.