This cheat requires you to be married and only works on spells that gain skill while casting while out of combat, i.e. Courage for Illusion, Healing Spells, etc.

Get married to someone and go to their house. Sleep for an hour, receiving the skill bonus for sleeping in your marital bed, then blast away spells.

Sleep again to quickly recover Magicka and renew the skill bonus for lovers comfort.

Rinse and repeat. I did this after marrying Camilla.