The goblins of the Steamweedle cartel have set up several neutral auction houses about Azeroth. Gadgetzan, Booty Bay, and Everlook all house neutral auction houses. The neutral is useful for making money, as commodities that Alliance players can get easily can be sold at a cheap price to Horde players (or vice versa), and then sold at a higher price at a major city.
Level with a pet if you can because it will help keep you safe. If your pet is tanking all of the damage from the enemies you’re fighting, you won’t have to. The other reason is that pets are easier and cheaper to heal than you are. If you don’t have a pet, make sure you know how to use your defensive abilities. You can’t level quickly if you are dying all the time.
Similarly, when earning by volume, do not overload the AH with your items. Basic supply and demand: too much supply will make your wares too common and unwanted. Furthermore, with great demand and a cheaper price, you may end up starting a price war with your competitors, with them actively trying to undercut and outdo you. Moderate the amount of items you're selling on the AH to small batches, refilling them only when sold out.
When assigning a selling price, do not aim for too low a price that would give you too little profit, but certainly do not assign too high a price. Too much greed is never a good thing, and the AH is filled with items that do not sell due to excessive inflation. A very common tactic in the AH is to sell for lower than what another player is offering, and many players make a lot of money that way. Similarly, losing sales thanks to being undercut is never fun. Aim for as low a price you can that will still make you a good profit. If you can keep producing the same item over and over, sell cheaply yet make a good amount of the item, you will reliably make a lot of money by volume. Moving inventory is the best kind of inventory.