Everyone has a way that they really enjoy making gold. This may be because they make a lot of sales, or it may just be plain clever.
My two favorites certainly have to be Jewelcrafting since this is where I make a ton of my gold.
The second for me is recipe flipping. This is a market that a lot of people don't really get fully involved in. I grab very nice underpriced recipes and relist them for significantly higher on a regular basis. I have made at least 50k profit off of flipping cata level recipes alone.
What kind of things do you like to do to make gold?