Most players think that tailoring just isn't a good money making profession on WoW, but you will be surprised at how easily you can sell items that don't make a hill of beans either way in helping a character, such as shirts.
This sounds dumb, but some players will go to any lengths to ensure that their armor and other clothing matches. You can sell silk shirts anywhere from 1g-4g for the darker colored shirts. But, if you can purchase a pattern for the "rich purple silk shirt" you will be surprised that you will sell them for over 3g a pop.
There are a ton of other shirts that are selling like hotcakes all over the servers, such as the "black swashbuckler shirt", although mats cost a bit more. You can also find a good rate on tuxedos and wedding dresses at times, you just need to check the AH for pricing.
Other items that aren't shirts that sell well are hats and boots. Some hats, like the "Admiral's Hat" have some mats that can be hard to find sometimes. And, some boots, like "Spidersilk Boots", have expensive mats. But, you will regularly find that no matter the cost of the mats, you will find someone out there who is wanting to buy them.
One majorly hot item right now that normally sells from 5g-10g is the "Blood Elf Bandit Mask". While this pattern isn't hard to find, sometimes the mats are, which drives the price up. So, this is always a good item to make when your tailoring level is high enough.
Mats are another area that you can make a ton of money on. Mageweave, runecloth, and moon cloth are three mats that are running at a high rate if you can find them on AH. So, if you have them, stock up when you find them and then sell on the AH.
Tailoring goes hand in hand with enchanting, so you can make special items that you can then disenchant for mats for your enchanting or to sell on the auction house. So, if you are going to pick up tailoring, you should try to pick up enchanting as well.