I use a spell-point system inspired by TFT.
Spells cost a certain number of points to cast and spell points can be used to increase their power (e.g., duration, damage, etc.).
A skill check is required to cast the spell succesfully. Failure results in damage to caster proportional to the number of spell points put into the spell. Failure of a highly powered spell can kill.
Spells are not forgotten once cast. A caster can memorize a certain number of spell levels and repeatedly cast the same spell as long as they have the spell points. The repetition though is not much, never more than 5-7 spells at the lowest power levels.
The spell point cost is scaled so the base cost of spells produces a slightly less powerful spell than normal. If the spell is powered up you get about the same number of spells per day as normal.
Spell power is not related to caster level. Spell power is increased with spell points which are gained upon leveling, or through magical devices, etc.
In practice, increased flexibility for spell casters, concern over chances of failure in risky casting situations, spells points allow more low level casting but do rapidly run out.