Now this is the clinger. You're level 38 and suddenly realised you need 70 more gold for your mount, which is coming up in 2 levels. Do not fear, there's ways.

For starters, take a gathering profession (or two). Mining, Herbalism and Leatherworking all bring in goods you can sell at the auction house. Check the prices to see if Copper Bars, Briarthorn and Light/Heavy Leather are worth anything at all.

Keep in mind that some of this stuff is needed for Reputation related quests for high level characters who'd rather be lazy and spend some money than farm it themselves. Look for Kingsblood, Heavy Leather and Iron Bars in particular.

Rogues need Swiftthistle and Fadeleaf for their skills, you can sell stacks of these for decent money.

Low level cloth sells well too. Linen, Wool and Silk can fairly easily be farmed by a lvl 30+ character and should bring some nice money in the Auction House.

Fishing brings money - Oily Blackmouths, Firefin Snappers and later Stonescale Eels are needed by Alchemists (and in the Tier 0.5 quest in the case of the latter) and bring cash.

With Sergeant Rank (third PvP rank) you get a 10% discount on everything you buy. Honoured with the faction you're dealing with gets you another 10% discount. Spending some time in the Battlegrounds might save you a lot of cash in the long run.

You can often disenchant items and make a profit on the materials. For example, I tend to buy high level blue items to disenchant them into Large Brilliant Shards. If those shards sell for 5-6 gold, I can easily spend 3gold on an item and still make a profit.

People are idiots. Honestly. That, and lazy. On a new server I recently earned 20gold by level 19... just by buying Herb Pouches in Ironforge (10silver from vendor) and selling them in the Auction House for 50s-75s. They sold on buyout every single time. [Note, 12slot bags on that particular server went for 1-1.5gold. This won't work on a more established server.]
Vendor bought limited recipes work just as well. Det tested it a while earlier. He walked into town with a few silver and from recipes sold in Ironforge, he had over 10g a week later without setting a foot outside of town.

On the people are idiots note, you can use the Auction House as a gambling hall. Buy cheap items and resell them for a slightly less price. (Never use the buy low, sell high rule. Since everyone else does, you'll never ever sell high...)
You can use the add-on Auctioneer if your memory isn't up to scratch on remembering all that. Gorny's made a tutorial about that specific add-on.

If you want an interesting read, check out kcma's threads in the Hunter forums. He started on another server, and was aiming for 1k gold and level 40 in 7 days /played. If contains a few pointers which you might find useful.