This is a cut'n'paste of my response on Tobold's MMORPG blog about the World of Warcraft auction house. Find the article here.
Controlling the market is key. I have the items I watch and I will buyout and low sellers and undercutters. Then I will repost at my price. I usually can bully out the undercutters because they are usually aren't dedicated AH watchers. They got some cash and that's all they cared about.
My keys to AH riches while still playing the game are as follows...
1. Pick your poison and stick with it. Don't switch back and forth between multiple items unless you really want to waste hours in the AH. If you are a disenchanter... stick with it. If you are a miner... stick with it.
2. Own the market every login. Take the few minutes to buyout every undercutter. This is tough at first because you probably will take losses on some purchases, but you are ensuring your future market.
Don't be afraid the mail people if they constantly undercut you. Don't be mad at them! Explain to them that you would like to purchase at a set price and see if they will stop using the AH. It is MORE profitable for them to sell lower to you because they are NOT losing money on AH fees.
3. Don't flood the market. You have bank space... SO USE IT! If you find the market empty and a demand for your goods you will have the "stock" in the bank to fill up the market... and usually you can charge a bit extra.
30 minutes a day can make you 100 gold a week easily and I've proven it time and again. Just the question of what to buy/sell... I'll let you figure that part out.