Another fun way to get magic xp is to go to Salarin the Twisted in the Yanille agility dungeon. You can only kill him by using strike spells, so take a fire bstaff and some mind runes. You hit 12's every time (except the last, which is a 10). There are a few chaos druids there to train melee on using your bstaff while you wait for salarin to respawn.
Salarin the Twisted drops the Sinister Key. This key opens a chest that has Torstol (the herb used to make Zamorak potions). The chest also contains 1 kwuarm, 1 irit, 2 harralanders, and 4 rannar (Correct me if this isn't right). When I go, I usually get 2-3 keys per 250 mind runes.
This is a really inexpensive way to get magic xp and awesome herbs.
It's also a little bit of fun and something to do that's out of the ordinary.
Ohhh BTW - you need a rather high agility level to get to Salarin, and a high thieving level if you want to use the back door to the dungeon