Keep track of older esoteric items that retain value.
Something like the Rep drops from Skettis are always good, can sell for 100-200 each during TBC TW week, and are extremely fast to farm up.
Same thing with Basilisk Eyes for the same reason.
If you have a high enough rogue and your willing to do the legwork, you can usually find someone looking to to “The Insane” who will pay you a lot of money to farm them Heavy Junkboxes by pickpocketing BRD.
Then there are exploits… err… creatitive mechanic use.
So lets say you que into SOLO LFR heart of fear from above mogushan vaults, clear all the trash up to the first boss, then leave without killing that boss by right clicking and leaving group (and apparently the 2nd boss trash as well). Then solo reque and do it again, and again, and again, and again to lockout. You’ll have a LOT of motes of harmony, which you can trade into ore to resell on the AH.
You can also bring a druid to ZG. Run in and go travel form. Hit Gub on his bridge for the fish barrel, then go kill the 2 Berserkers on the way to Mandokir. Gub drops something like 20 Cata fish, and each Berserker drops 10-20 Cata ore and a few gems. Then use your dreamwalk to leave, then again to return to just outside the instance.
Reset and repeat to lockout. It’s much faster than farming ore and it still goes for quite a bit sometimes.