Have more than one character.

One of the most important things to make an enormous amount of gold on World of Warcraft is to have more then one character. You can only take two professions per character means you can only expand in two markets. If you have various character means you can do various professions daily's.
Let's take Jewelcrafting and Alchemy. If you have 5 level 65+ characters with all Jewelcrafting/Alchemy you could do the following:

• 5x Icy prism = 5x chance on epic gem
• 5x Daily Dragon Eye = 500g per day
• 5x Epic gem transmute = 750g per day without the proc. If you have 1 Elixir Master and rest Transmute you could even make more gems per day. I take 150g per epic gem, but with your Jewelcrafter you could make gems that would probably sell for 200g each that would be 1000g per day doing almost nothing.

This is an guaranteed 1500g per day wich is 10,500g per week only spending 30 minutes per day. That's an average of 2300g per hour!

I don't really recommend taking only Alchemy and Jewelcrafting but try to split all crafting and gathering professions between your alts so you can compete in any market.