1) Get a decent level of Magika (probably no more than 100 needed). Train Illusion to level 25 on your own, and get the Novice Illusion and Apprentice Illusion spells.
2) At some point before you get to level 25 Illusion, get into the College of Winterhold.
3) Buy the Muffle spell.
4) Talk to Drevis Neloren in the College and ask for work to do.
5) Complete his really easy job, and at the end, you'll have a +100 Fortify Magicka bonus for two hours. Now comes the fun part.
6) Dual spam the hell out of Muffle to your heart's desire, your Magicka will regenerate instantly.
I got to level 100 in an hour.