Krick has a lot of good information on using Auctioneer. My friends who introduced me to WoW have always made fun of me because I like the market so much. Last summer I played for 3 months, started a new character on a server with no friends to give me monetary support. Using Auctioneer I ended the summer at lvl 69.5, with 6.5k gold in pocket and had over 30k go though my pockets in those three months. I was buying every epic gear piece I wanted, all the best enchants and gems, as well as all mounts as I hit the level. I was a twink, but with no backer, all because of Auctioneer.
So, enough of my story. To give you my secrets! Not really all that secret, Krick hit a lot of them. I would recommend selling common items, I held onto some epics for months, because no one was buying. Never did sell some of them, I gave them away eventually, I got so tired of putting them in AH. Metal and cloth are the best, but leather and herbs do well too. At least on my servers.
Many of the metals do better if you smelt them, such as gold ore, but one like Silver ore, or Truesilver, which are bottlenecks in the smelting leveling, sell for big bucks. Such that I normally sell silver 1 by 1, rather than any stack. Other good metals are thorium and mithril. Mid range metals, so what were high in vanilla, sell well.
Wool and above sell like mad. I never get any runecloth back into my mailbox, they sell so quick. Buy cheap, sell high. People will buy!
With leather in Wrath I figured out a cool scheme. Buy borean leather for cheap, and then convert it into heavy borean. I would normally sell them for small stacks, (5x), because when I was lvl 12 I wanted one of the big bags, but didnt have the gold to buy a full stack of heavy. And it sells better in smaller stacks, because that is the case with many people. You have to do a little math to make sure you are making money, (20 borean in a stack, 6 borean for each heavy, how much are you making?), but it worked like a charm. I get leatherworkers to change a couple hundred for me for 20g, and sell it easy.
The best leathers to sell, I found, are the main ones, heavy, light, medium, borean, etc. The weird ones, like scales and tiger leather, dont sell well because they are only used in a few rare recipes.
I gave up on selling herbs, but that was because I have a scribe and alchemist alt. Fun to have a lvl 10 with maxed professions.
Depending on the server, bags are amazing. My tankadin server is crappy, other people have that market, but my shadow priest does good trade in them. With those, find someone with all the different recipes. Leveling characters dont have the money for the biggest bags, so the smaller bags from vanilla and BC are great. They require a lot more materials than Wrath bags, but they are easy to collect if you are looking for them. I love making a dozen soul pouches and selling out in a week. Costs less than 12g to make during a bad week and sells for 20g on my server, while the abyssal is in the 400g or more. I personally love the Cenarion Herb Bag and Satchel of Cenarius. But, you have to have someone with the recipes. Trade chat works if nothing else.
Basically, sell what people need. What EVERYONE needs. There is a demand and just make sure you are the supply. On my tankadin and my shadow priest my name is on most every page of the Trade Goods tab.
Use Auctioneer. It is your friend. Good luck making money!