First thing you MUST do is download/install the auctioneer addon. This addon will save you tons of time in the future as it will help you find items that are selling way below actual prices. This is by far the best addon I have ever seen. (Probably because I spend 90% of the time in game at the auction house!)

Once you have auctioneer installed you will want to just scan the auction house a lot for about a week to get accurate values on items. By a lot I mean no more than 3 times a day. Try to do morning, afternoon, and then again in the evening. What's that you say? You have to work? Well, you do not want to scan less than once per day. I find myself only scanning 2 times a day because I don't have a whole lot of free time anymore to play.

Now that you have at least a week worth of data you can start finding things to buy for pennies while you sell them for gold!

There is a function in auctioneer that will tell you what items are selling for way cheap and you can even specify for it to show items that will make you 5-10 gold if you want. Probably one of the BEST features of the auctioneer addon! Once you do that you will also want to do a manual search on the auction house to double check prices and make sure that the price wasn't dropped because of that item being flooded on the auction house.

You will also want to check to make sure how many times the item has been listed on the auction house since you started scanning, just to make sure someone originally didn't jack up the price when it shouldn't have been. On a side note, this will not be the case too much after you have scanned the auction house everyday for a month or so.

Now that you found an item and bought it you are going to relist it! There are some simple rules to follow when listing items on the auction house. First, you ALWAYS want to list items for 24 hours any less time and you are wasting money. Second, ALWAYS set a buy out price on items. Think of it this way, when you found an item you want for your character on the auction house and the item doesn't have a buy out price, what do you do? If you are like 99% of the players online you want it NOW! If you can't have it now you look somewhere else. Am I right? Third, ALWAYS set your buy out price just a little lower than the competition. We want to sell the item as soon as possible and we don't want to have to relist it unless it is a very high ticket item.