One of the best tips available for earning more gold in WoW is to get the Auctioneer addon. This handy tool learns the auctionhouse, over time, and will give you valuable info on the market price of any item you come across.
You can grab Auctioneer here, documentation is available on the site. Hit the site and go to downloads. You’ll want to full suite, which combines both the classic and advanced suites.
Here’s a video which will show you how to make wow gold by using the various Auctioneer functions. Check it out.
Update 10/24: Auctioneer has officially moved to the advanced suite and the “classic suite” will no longer be supported.
The buying and selling is now done through the Appraiser interface, which takes a little bit to learn, but which is actually kind of cool.
You can either dig through the docs and forums on the Auctioneer site to learn how to use it, or you can log into your Massive WoW Gold Blueprint account, where Jon has uploaded a 45 min video on the ins and outs of the new Auctioneer. Also included with that are 45 minute videos on the Inscription profession and the buying and selling of herbs for massive gold.
If you don’t have a Blueprint account you can check that product out, here. It’s wildly different from all the other gold guides out there: The Massive WoW Gold Blueprint Review
edit 6/26: Keep in mind that as Blizzard updates WoW it also updates the programming code used to build our addons. This is what results in addon breakage and the need to update our addons. So if Auctioneer isn’t working (see comment) upgrade to the latest version.