Playing the Auction House: Professions
Play the auction house. Of course, we've all heard of it. But how do you do it? There's a number of ways. If you're an enchanter, you can buy rare (blue) items for 10g or under, disenchant them and sell the shard for double the price. Tailors can buy up cheap stacks of netherweave and make them into imbued bolts, or spellcloth, shadowcloth or primal mooncloth! Each profession has it's "sweet spot" so-to-speak. Many different items have relatively cheap mats, but a high cost for the finished product. Buy up the mats, make the product, double your money!
Playing the Auction House: Economy
The second popular way to "play the auction house" involves knowing a lot about your particular server's economy. This, basically, involves watching the AH like a hawk to find items that are priced particularly low. This happens fairly often, as many people will just guess at a price for some items - and tend to guess wrong! You, knowingly, will snatch it up and sell it back for a higher price that you know it will go for.
Farming for Primals
Everybody needs primals, and will for a good long time. Depending on which primal you are farming, you could make 30g for 10-15 minutes of work! The best way to do this is to use a popular Warcraft item search site, such as Thottbot or Wowhead, and look up where each mote drops. Your best bet is to find a place that isn't heavily farmed, and has mobs that are lower than level 68-70. This way, you can kill everything fairly quickly.
Can't find a place that isn't heavily farmed? Well, it's just a matter of finding the right time. Hit up your favorite spot early in the morning or right after the servers come up and you won't be met with much - if any - competition. With nobody around, you'll notice much higher drop rates and constant respawns.
Gathering Professions
Another popular one. Pick up mining and skinning, or skinning and herbalism (not mining and herbalism, as you can't track both at the same time) and farm until your fingers are falling off! Not only will you make money by selling the items you end up with, but you'll save money by not having to worry about blowing $1000+ gold for those "last few points" in the tougher professions. In fact, you'll make a lot of money off others who *are* looking to level said professions, since you'll be selling all the materials they need!
Professions in General
As previously mentioned, leveling the tougher professions takes A LOT of time and money. But in the end, it can be worth it, especially if you're willing to grind out the reputation to get some of the rarer patterns or schematics. Making these rare items to sell on the Auction House, or just setting yourself up a macro and spamming the trade channel, can make you a lot of gold. Just don't forget that the key to it is farming most of the materials yourself instead of buying them, otherwise it may not end up worth it in the end.
Farming Reputation Items
Ah, reputation grinds. Nobody likes it. That is, unless you're profiting off of it! Make a list of all the reputation items people are looking for and farm them in your spare time. Things likes coilfang armaments, glowcap, fel armaments and arcane tomes all sell fairly well - and fast!
Run Lower Level Instances
Now that BC is around, nobody is running the level 60 instances. Of course, being level 70, you can run them mostly solo, if not with one or two other people. It will be quick and easy, netting you a nice amount of gold. Plus, if you're a skinner, miner or herbalist you can pick up some materials that will sell for more than you think. Being an enchanter helps as well, as you can disenchant all the drops and sell the shards.
Why would people be looking for these materials NOW? There's a major shortage, as nobody is running these instances, but there's still a demand, whether it's for lower level alts, or level 70s who can't afford the upgrades. They'll sell well and most likely for an inflated price.
General Farming
I'm putting this as one big item, because it's all the same idea. You can farm humanoids for cloth, all of which sells for 3-4g per stack on the AH. Or, you can farm for rare BOE patterns or schematics to sell to those who don't want to put in the time. Hunting down some lower level elites may not be a bad idea either, as they drop a higher amount of gold.
OnMyList
List your pants off!