Deviate Fish and Savory Deviate Delight

With Deviate Fish you can earn money very simply at low levels - especially with the Horde. All you'll need is 85 skill points in cooking (you will have those within 5 minutes) and 30 skill points in fishing (just fish aroung in the starting areas and it will be quick).
Then head to the Wailing Caverns in the middle of the Barrens (a bit west from Crossroads). Just go in a bit - NOT in the instance itself - until you'll reach a small lake. Hide in a safe corner and just start fishing. About every second fish there will be a Deviate Fish.
You can sell the fish as it is, but for much more money, we suggest the Savory Deviate Delight. Just get the recipe for "Savory Deviate Delight" from the auction house or by farming monsters in the Barrens and you can cook an expensive meal. Players will pay quite much for the Savory Deviate Delight, as it will change their characters into ninjas or pirates.

Selling Recipes and Books

With this hint you can make real profit through all levels. All you have to do is buy recipes and books from npcs and sell them for much more gold in the auction house. Many players don't know where to buy all these recipes or just don't like the long way.
Here's one example: The three First Aid books in Arathi Highlands. You'll find Deneb Walker (Alliance) in Stromgarde Keep. Just turn right after the entrance, then sharp right again around the corner, over the bridge and there he is, willing to sell you the following books: "Expert First Aid - Under Wraps" (cost: 90 silver each), "Manual: Mageweave Bandage" (cost: 45 silver each) and "Manual: Heavy Silk Bandage" (cost: 19 silver 80 copper each). For the start, let's buy 5 pieces each. Then head to the next auction house. You will sell them for at least double the price they cost (Manual: Heavy Silk Bandage is often 2 gold each!). For the Horde, head to Brackenwall Village in Dustwallow Marsh and buy the books from an ogre called Balai Lok'Wein.
Just keep your eyes open for recipes (or search for recipe and similiar in WoW-Databases) and buy lots of them, for you will sell them for double the price in the auction house later on.

Buy and Sell - Big Bussiness in the Auction House

Somewhat more risk but quite efficient way of making gold is to search the auctions house for special "offers". Buy items that are offered for below their price or cheap ending auctions and sell the items for much more later on. For this you have to know the prices exactly and visist the auction house quite often.

At last you can get rich by building up a monopol in the auction house. That means if there are only a few (quite cheap) items of one kind in the auction house, buy them all and resell them for much more gold. Nobody else will have one of those items in the auctions house (because you bought all of them), so you can sell completely unrivaled.