I am sure everyone has seen those guys with thousands of different items in the auction house. They don't necessary spend half their day on the AH they just know the better items to sell and when and how to sell them at the best times. Through lots of time and practice you can really get a good understanding of what items are selling and whats not going to sell. Lots of the time you will be able to buy and resell other peoples items at a huge profit. In fact that's where you make the bulk of your gold.
ADDONS - I can't stress enough on these, there our addons for the AH that can make your buying and selling on the AH ten times faster than manually scanning the AH yourself.(Check the link below for a list of addons)
Bank alts - Set up a toon or maybe more to store all the items you have. I learned from experience that buying and selling on multiple characters was really time consuming and confusing. Make the character and keep all your AH bought goods on him to buy and sell.
Seasonal Items - Stock up on those seasonal items in six months some of that stuff will be selling for ten times what you payed for it.
Professions - They can be a huge money maker especially if you have any rare recipes. Now I like gathering professions for that constant stream of gold but something like jewel crafting made me tons of gold when wrath came out just buying the mats from the AH and making the items with even normal recipes.
Patience - This is a big one some of your stuff wont sell in an hour. I have stuff that doesn't sell for weeks but you just need to keep trying till you learn the market. Plus some stuff doesn't cost anything to put up like enchanting materials.
Undercutting - Don't just buy everyone's mats then resell it for double the price. The market for the items will just be flooded. Buy the stuff that will make you only a profit for the recommended sell price.
Those six steps will help you get started on understanding how to use the AH to you advantage. In the box below is a more detailed guide.