This glitch? is pretty simple. First acquire a horse. Second,, acquire a soul trap spell, third, shoot the horse with the soul trap. Repeat Part three for about 40 minutes. This is good because at 90 conjuration there is a special quest you can do.

You can also cast it on a corpse to level. Also something I found humorous on UESP:

"Another option is to find a Horker up in the north, equip Bound Sword in both hands and walk circles around the Horker continuously casting and recasting the spell. The Horker is too slow to catch up and will keep trying anyways."

"Thats no horker - thats my wife"
Cicero

Or just play the game using conjuration, levelling it naturally, crazy idea I know?!

I imagine you could do it even quicker using the magicka regen boost given after the 'cleanse the focal points' quest is done, you regen magicka stupidly fast and even when your casting for 2 or so hours.