Hi there :-)
For a night elf mage, I would say tailoring. It seems pretty useless until you get to about level 85, but when you do you can make heaps of cash with patterns that every cloth wearer wants, and also craft your own gear with no extra expense.
As for a second profession one that goes rather well with tailoring is enchanting. Considering you loot cloth of humanoids to craft green / blue gear which you probably wont use, instead of just selling it disenchant it! This works all the way up to level 525.
Good luck

~Chris