In case you want to be REALLY good at magic, and you want to do it fast, then you can try this tip.

Let's say you want to increase your Destruction from 5 to 30. Fist thing you do is buy yourself a Destruction spell, for instance Fireball.
Then you find yourself a person with Spellmaking (Mages guild in Balmora is excelent).
Make yourself a Firespell that only work on SELF, set the damage to "0 to 0" and call it something like "Destruction training".

Now find yourself a bed and then use the spell until you are drained. Sleep until healed, and repeat.

It took me about 30 min to raise from 5 to 20 this way, and that was a miscelaneous skill. Sure, it takes some time, but it's quicker than adventuring.
And the best thing? Next time I made a level I had a *15 on my Willpower! ^_^

Yeah, I know. It's lame and shows I have no life! ;)

Great tip Gormash! I have raised some of my attributes in this fasion also. I bought a "wild open" spell, found a chest with 100 pt difficulty and spent days (game time) casting it at the chest. There was a bed right next to it so no problem resting and regaining my magic.
Another couple of ways I raised some points is buying "Mark" and "Recall" teleportation spells, and conjuration of a flaming atcronach(sp?) I mark spots near guild guides and within seconds I can be on the other side of the island while gaining points in this attribute going back and forth. Conjuring works the same way.

*\Conan/*

correction you cannot gain more than 5 points per lvl on willpower per sleep. so you would have to gain a lvl while gaining only 5 points in willpower. unless you wanna waste all your other 10 points needlessly.

I know. It was just a typo! ^_^