Once you get them, you can just "recite" them to cast the spell, taking mana.
Once you use it, you learn it, never forgetting it.
Once you use the magic once, you learn it, but after a while of non-use, it is forgotten...
I actually have a good idea on how to do that now!!
Man am I smart!
Basically, using list elements, you start out @ 0, and if it gets used, that particular spell gets +1 usage, but if it is not used, it gets -1 usage. Once it reaches -10 or -15, you forget the spell. I think I am going to make the special spells immune to that effect though...