Mining, Herbalism, and Enchanting are all money makers.
Mining - Metal is the most needed material in WOW. 3 professions use it all the time (Blacksmithing, Engineering, and Jewelcrafting) and other professions need it occasionaly. Ive see stacks of copper going for over a gold. Easy money.
Herbalism - Find out the herbs that are hard to get, needed for popular potions, or needed for speed leveling Alchemy. In your mid-20s certain herbs sell for 3-5 G a stack and it just goes up from there.
Enchanting - This one is tricky. You dont make money from enchanting stuff until you hit 300+ skill, but you can make sick money from disenchanting things and AHing them. Buy cheep greens from the AH, disc them, and AH what you get. Trick with this is that you have to level your Enchanting enough to be able to disc the high selling stuff.
Devious gold making from the uneducated: go to lvl 10-30 zones (barrens etc...) and buy materials for super discount prices from people that dont know better. Proceed to the AH and make bank. Beware, this works on weekends when a lot of noobs are on and can take more time than its worth.