I decided share one hell of a secret with you. First off I need to let you know this one has a bit of a catch now. It takes 2 accounts. This isn't much of a problem, tons of people have more than one WoW player in their house now, or a trusted friend they play with.
This technique is Cross Faction Auction House Trading.
I am sure you know that the AH is your key to making tons of gold. No matter how much you are farming mobs and getting rare drops, it all depends on how well you can manipulate the AH. It comes down to Supply & Demand.
What a lot of people may not really think about is that there is a huge difference in S&D between Horde and Alliance. Prices that are sky high for stuff on Alliance AHs may be rock bottom on Horde AHs. This is the case on every server, but the particular items will be different on different servers.
Setting Up Your Pipeline:
Make low level alts to do all of your buying and selling.
You need 4 alts in total, 2 Alliance and 2 Horde. One Alliance toon sits in Stormwind, one Horde toon goes to the Undercity, and you need one alt of each faction at the Neutral AH in Booty Bay.
You can choose a different combo of cities if you prefer, but this is the one that I feel is the easiest to get going. Stormwind and Undercity also have more convenient access to some of the Limited Supply Items I have told you about already.
Be careful setting this up on PVE and RP servers. Even though you are allowed to create both Horde and Alliance toons, you won't be able to purchase your own auctions w/o a second account.
This won't be an issue on PVP since you can't make both Alliance and Horde toons on the same account and server.
For the main AHs, you don't even need to level your alts up at all. For Horde, make an Ogre or Troll and run them north to Orgrimmar, then fly to the Undercity. On Alliance, make a Human and run it west to Stormwind.
You can level them up a few levels if you want, just to get a few silver on there and have a bit stronger character to make the run faster... but this is up to you.
The Neutral AH is a bit tougher, since you have to go through some kind of dangerous areas. This is why I recommend Booty Bay as opposed to some of the other cities.
For the Horde:
Create your toon, Troll/Ogre, and then run a short way south from the starting point. Then head south/southwest into the little flatland patch you see on your map, towards the western coast of Durotar. Head SW across the flat area and up the "Hidden Path" you find at the south-west corner (past a Lazy Peon). Keep following the path out to the coast, jump in the water and start swimming west. A Slimeshell Makrura will probably kill you right away. This is good though. When you release after dying, you will be at the Ratchet graveyard in the Barrens. Accept a rez from the Spirit Healer and run down to the dock in Ratchet, NW from the graveyard. Take the boat to Booty Bay.
The Booty Bay AH is right at the Bank which is at the far opposite end of the wharf (south) from the boat landing. You will find Auctioneer O'reely standing on a crate between the bank and the mailbox.