The most important thing to know if you are trying to make gold is to know the tools and resources that are available to you and how to use them. I am not trying to make an inclusive list, just my personal preferences.
If there is one resource that I think is more important than any other and that everyone should use is the web site The Undermine Journal.
Use this to figure out what the value of items you have found are really worth. Think something is too expensive or is a really good deal? Check here to know that answer before doing anything. This site also has blogs and posts of ideas and opportunities for making gold. Some can actually be quite fun and class specific, IE Pickpocketing on the Isle of Thunder.
Similar to Undermine Journal is WoWuction. http://www.wowuction.com/eu/bloodhoof/horde/ This site presents the AH data in ways that differ from Undermine Journal.
Another great online resource is The Consortium
There are many great forums and links to other online resources. Stop here to explore ideas and look for tips.
If you are wanting to try your hand at using your professions to make gold but don’t know where to start? (which gems should I cut?) I found that WoW Popular shows some good lists of the top crafted items.
There are many bloggers and podcasts about on gold making that can provide far more information that I ever could.
Of Course there are other resources, these are the ones I found most useful to me.
Trade Skill Master (TSM)
TSM is hands down the primary addon for working the auction
house. A must if you are to be a power seller. It has a pretty steep learning
curve compared to other auction house addons, but there are so many benefits it
is worth the effort. As a side note, TSM 2.0 is currently in beta and will be changing how it works. Just be aware that you may end up learning TSM 1.0 just to have to re-adjust your thinking for 2.0.
Auctioneer and Auctionator
I have used both of these, I have always preferred Auctioneer over Auctionator, but they are both good. If you are even going to attempt to use the AH to make gold you need some sort of AH addon. The default UI is horrible. Blizzard has even publicly acknowledged that the WoW AH is out dated and lacking, but they feel the addon community has provided the missing features and have no intention of improving it. If you use the AH, get an addon,
Mail box tool that lets you collect the bulk mail.
Makes buying stacks of vendor items easy.
Like to farm ore, plants, fish, or just about anything else in game. This addon is an easy way to live track how much you have acquired.
Lets you right click on items in your bag disenchant, prospect, mill, etc.
Gold Making Pod Casts
There are quite a few, there are two that I have settle on.
Jim Younkin runs the site Power Word Gold and does a nice podcast. He has lots of good resources on his site.
The other is Call To Auction. http://calltoauction.blogspot.com/ These guys are an economist and a mathematician who apply their professional trade and ideas to the making of gold in WoW.