So for my first real post I thought I'd do something a bit different. This is something I've not seen discussed, possibly because it is a bit self-evident, but it is worth mentioning nonetheless. There is another way to "make" gold beyond the normal buying and selling, and that is in the money you save by being able to craft your own gear / gems / enchants / flasks, etc.
One key thing acutioneers do is buy stuff off the AH or other players and turn it into something desirable that they sell right back for profit. The simplest, and my favorite, example is Jewel Crafting. Very few people want those nasty worthless dull raw gems. But everyone wants a shiny sparkling gem that makes you more powerful! So the JC buys the raw, cuts it, and sells it right back for much more than he paid, and that's how gold is made.
But unless all you do in WoW is play the AH, you probably want this stuff too! And if you're like me, you'll be damned if you're going to pay those ridiculous prices on the AH. So what do you do? You make it yourself! This also directly ties into why it's always a good idea to have alts with diverse professions. With the right combination of professions, you'll never have to pay more than the price of mats for anything your character needs. And that money you don't spend is more money you have.
I'll end with another illustrative example. I have three professions I like to focus on: enchanting, jewel crafting, and alchemy. Once I start raiding, because I have alchemy I'll always have pots and flasks at the costs of mats. This is quite obvious, but more interesting is how the three will work together to almost guarantee that the cost of ore will be my go to for gems and enchants. From ore I get raw greens. I can either turn these greens into blues with alchemy, or enchanting materials by making rings / Spikes / necks with JC and disenchanting them. With enough ore I can do all this and, if prices stay in near the same ratio, still come out with a profit on top of getting freshly gemmed and chanted. Of course, there will be times when ore won't be the cheapest option (e.g. raw blues are worth almost as much as one raw green right now and I still haven't found a reliable way to DE for shards...), but it's always an option to look into.
So when you're out there making gold hand over fist, take a second to reflect on how much gold you're actually saving when you're making stuff for your character.