Casters use the memorisation tables in the rules they’re currently using. For an OD&D magic-user, for example, this means 1 first level spell at 1st level, 2 first level spells at 2nd level, etc. They also say which specific spells they have prepared, just like usual.
But!
Their prepared spells are just the ones they have at the front of their mind, ready at a moment. They can, instead, cast any other spell they know of the same level. Have Sleep prepared and want to cast Charm Person instead? No problem, but you will have to successfully save vs. spells. If you make the save, no worries. Fail the save, and you have to roll on a spell failure table.
What if your 2nd level magic-user has already cast two 1st level spells and needs to cast another? Same thing, but it will also take double the casting time to get it off.
This also works if you want to let spellcasters attempt spells which are beyond their current ability, something which happens in the stories now and then. I'll apply penalties to the save vs. spells depending on how far beyond their abilities they’re trying go. I wouldn’t allow a 20 to always succeed in this case, either.
One more thing I’ve decided. A character can only benefit from one casting of any specific healing spell once per day. Once Dilbod has been healed by a Cure Light Wounds, the magic needs some time to sink in before another Cure Light Wounds will have any effect on him, regardless of who casts it. A different spell, like Cure Serious, could help him some more before tomorrow though. This may be fairly harsh, but I think it’s necessary if I’m giving casters a way to exceed their limits. I’ll allow people to bind open (only) wounds to stop bleeding and get back 1hp, once for each combat in which they get themselves hurt.
Subject: Spellcasting Rules 
6th February 2012, 2:32 pm
