I've had a nice two week break from work over the holidays and the guild had a raiding break as well.
When I've played I've done a few heroics and silly achievements but I've also been spending some time in the Auction House. Without doing anything fancy really I've gone from about 8000 gold to 21000 gold. I realize that this is something which some people can make in one day but for me it's a fantastic result.
As preparation I've been reading a bit on these two nice sites:
Woweconomist and Greedy Goblin.
It's quite simple really, I've basically sold these in large amounts:
Glyph of Death Strike: 17-20g
Glyph of Plague Strike: 17-20g
Scroll of Enchant Gloves - Exceptional Spellpower: 115-150g
Scroll of Enchant Boots - Greater Fortitude: 95g
I turned my Death Knight into a Scribe/Jewelcrafter instead of herbalist as my main gets enough herbs as is and the outland herbs are cheap enought to buy on AH. This lets me prospect Northrend ore to get blue gems to sell. I can craft high level items to disenchant for mats to my enchanter who can sell various scrolls which I can make the vellums for. I turn my cloth surplus into spellthread or imbued netherweave to sell. I bid a lot on AH greens to disenchant.
The main money maker is selling the scrolls from enchanting and the glyphs for the death knights, it's a solid steady income. Then there's also money from selling herbs, cooked fish, the tailoring output, prospected gems and buying and relisting trade goods that's listed too cheap.
I also never flood the market with anything but only sell about 3-4 of each item at a time and from various characters.
Once you get enough money you can have a lot of pending transactions which all adds up. When I log in to empty the mailbox on a character I have to completely empty it 3-4 times because all the incoming auctions can't fit at once.
Herbalism, Inscription, Jewelcrafting, Tailoring, Enchanting, Cooking, Fishing + patience and knowing the prices -> profit =)