Over the past year and a half, Vicky and I have accumulated and are sitting on over 3000 gold between our mains, alts and mules on Terenas alliance. (And both our mains have already purchased epic mounts).
This comes from many sources:
* At least a third of it comes from "playing the market" (when I login and don't have time to quest/instance/raid/grind). This is purely me.
* Around a third of it comes from simple gathering (mining, herbalism, skinning, disenchanting) that comes naturally while questing, leveling, etc. Both Vicky and I do this.
* The rest of it is miscellaneous: selling every gray item to a vendor (instead of letting it rot on a corpse), gold that comes from killing humanoids (especially in instances), not buying anything for our own use (gearing up via instances instead), etc, etc, etc.
Long story short: it's not hard, and there are multiple routes. The most common path, IMHO, it the simple gathering route. Herbalism, especially. Dreamfoil is king!
To save time, I just mail everything that is AH-able to one of several of our mules (categorized), so my main/alt doesn't have to worry about dropping into town to sell stuff. Our mules alone probably have over 20 days /played time collectively, granted, over 1.5 years.