I'm not sitting on huge amounts of gold atm, but that has more to do with getting enchanting and inscription to 375 than anything else. Plus, I'm lazy. But in the interests of being helpful, I'm going to share how I make my gold...
1. tradeskill cooldowns. Do them. Every day. Go get the mats if you need to, but do them.
2. Cutting epic gems. I use wow-popular to decide what cuts to get, buy gems when they're relatively cheap or transmute them via my alchemist, then cut them and post. Plus the epic gem market on my server is quite a bit less volatile then the regular gem market.
3. When leveling tradeskills, I vary what I make. It's much easier to unload 2 glyphs of horn of winter and 2 of shadow word:pain, then it is to unload 4 of one. Especially since there's a lot of competition and volatility in the glyph market. Oh, and always have the vellum for leveling enchants. Always. Lower-level enchants can and do sell, some of them for very nice chunks of change.
4. Alchemy + Jcing is win. Add in mining and it's double win. Alchemists make metagems and epic gems and titanium bars from saronite. Jewelcrafters can provide most of the raw materials for these transmutes.
Jewelcrafters can cut these gems for max profit, and a miner can use the titanium to make titansteel. Plus alchemists can transmute eternals from one type to another, which is just handy.
I occasionally quest (a good moneymaker), farm ore/herbs, or do dailies, but those are all more of a "I'm in the mood to do that" sort of thing or "man I need that for something", whereas I do my cooldowns every day and check the gem markets every day.
Oh, and running dungeons can also bring in a nice bit of cash, just pick up all the greys. I'm always surprised at the number of people who leave the greys behind. They add up quickly, and there are even addons that will sell them for you as soon as you interact with a vendor.
Now, these tips will probably not make you a wow millionaire or anything like that, but they will bring in some nice cash. Now if I could just remember that quite a lot of my gold is tied up in inventory right now so that my gold stores didn't look so puny, I'd be happy.