I use a simplified spell point system. Think of each slot a caster has/day. You can cast any single spell of the level of the slot or lower with that slot. This works one-way; no combining low level slots to cast a higher level spell. This is balanced by spellbook and spell component rules plus hefty spellcasting dues collected by the guild. Clerics may find that either their deity or their church place limitations on their spellcasting if they abuse it.

Example:

Skeeve is a fifth level wizard. He has 4 1st level, 2 2nd level and 1 3rd level spell slots each day.

He could cast 4 1st level, 2 2nd level and 1 3rd level spells in a day.

- or -

He could cast 7 1st level, 0 2nd level and 0 3rd level spells in a day.

- or -

He could cast 5 1st level, 1 2nd level and 1 3rd level spells in a day.

- or -

He could cast 4 1st level, 3 2nd level and 0 3rd level spells in a day.

Or any permutation thereof.

He could not cast 1 1st level, 2 2nd level and 2 3rd level spells in a day.

There is no need to specify which spells that a character memorizes each day. Mages may cast any spell that was in the character's spellbook(s) that they studied from that day that (providing that they still have a slot remaining of the appropriate level or higher). Clerics may cast any spell from their spell list (providing that they still have a slot remaining of the appropriate level or higher).

Time must still be spent by wizards studying spells to regain spell slots.

Clerics must spend one full uninterrupted hour on prayer to regain spell slots, regardless of how many spell slots they wish to recover, and can do so only once per day.

Any spell slots not used that day are retained in memory for use the next day.

The cantrip list is from Unearthed Arcana.

Wizards can cast up to 4 cantrips/day/caster level. This does not count against memorization time; but you still need to have the spellbook with the cantrip in it to refresh your memory each day.

If that´s not enough, for each spell slot used, you can cast a cantrip.

To balance the large number of cantrips/day, certain more powerful cantrips, such as unlock and present will be made to be less powerful. Also, only one cantrip can be cast per round. Cantrips (even person-affecting ones such as Belch and Yawn) will never disrupt the concentration of a spell caster.

All cantrips have a casting time of zero (0) and have a range of ten feet.

Orisons (priestly cantrips) are not allowed. Clerics already have significant spellcasting advantages and I feel that cantrips should have a more ´wizardly´ feel to them. For similiar reasons, bards can not cast cantrips either.