FWIW, this is how I do things in my Basic D&D campaign

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* The number of spell slots represent two things (1) the character's learning, and (2) his power.
* The character may prepare a number of spells equal to these slots; this represents especially dedicated study. Multiple slots may be devoted to a single spell to represent heightened specialization. This requires access to a magical library, spell books, etc...
* The required INT for casting a prepared spell is 9+Spell Level--the chance of failure is 5% per point below this.
* The required INT for casting an unprepared spell is 18+Spell Level--the chance for failure is 5% per point below this.
* A wizard may attempt to cast a spell by sacrificing one of the daily spell slots.
* On a failed attempt, a prepared spell loses one slot devoted to it, becoming unprepared when no slots remain; while an unprepared spell may no longer be cast at all, until the wizard can study his tomes.
* A spell remains in the prepared category until such time as failures move it out, or the caster chooses to prepare another spell in its place, and may be cast multiple times during the day, regardless of the number of slots devoted to it--subject to the daily spell allowance and failure rolls.
* Similarly, unprepared spells may be cast multiple times, until a roll is failed, or the caster runs out of spell slots for the day.