PLEASE NOTE: This may get you banned. Blizzard doesn't allow in-game gambling anymore, so use at your own risk.
Basically, they make a bet, and /roll. If the roll is in the brackets (which will be explained below), they win.
Step 1: I usually make a new alt (Using your main character isn't good, i tried it, got flamed for days :/).
Step 2: Give the character about 50-100g to start off with, something around that.
Step 3: If you are Alliance, go outside Stormwind AH, if Horde, go ontop of Org Bank.
Step 4: Make a macro saying something along the lines of this:
Want to give gambling a shot? Why not have a shot with me. Roll (73-81 Win 2x your bet) Roll (83-91 win 3x your bet) roll (96-100 win 4x your bet).
Step 5: Now, say they bet 10g, and roll /96, you owe them 40g. Don't logout, pay the money. If you pay them, the higher they will bet
Step 6: Start spamming the macro a little bit via /yell. More and more people will come.
Step 7: The gold does rise a lot, i've had my good days, and i've had my bad, but in the end, i'm always making a profit.
I hope this helps, it really is a great way to make your gold exceed.