If you don't care to take up two gathering professions, then skinning and leatherworking go hand in hand. While leatherworking is yet another profession that doesn't make you a ton of gold while you are in the lower level stages of it, once you hit the higher levels, you can easily make some gold.
For example, any type of armor kit that you can make will always sell well on the AH. But, here is the trick to selling them - only do them in stacks of 5 as most players will need 5 to do all of their armor. Then, when they upgrade, they can purchase another 5 and so on. You don't want to list them in stacks bigger than 10 though, as many players will automatically assume that is too many and they don't need them.
Now, until you get past the 250+ mark in your Leatherworking, you want to try to farm double mats and here is why: instead of leatherworking all your mats, the best way to make gold off of them is to create stacks of 20 of mats that are in high demand. Ironfeathers are one such item, and it takes a lot to make certain items. There are some bracers that use 60 feathers, shoulders that use 80, and the Ironfeather armor breastplate that takes 120 feathers. But, don't stack them in 50, (these are one of the few items that will stack in that high of an amount), instead stack in 20. This way, you will sell more to those who want the bracers or shoulders.
Basically, there isn't a lot of gold to be made leatherworking until you hit a very high level of it. But, with some of the awesome patterns that are coming out of the different areas of Northrend, once you hit the 350+ mark, you should easily be making some great gold with this profession.
Just make sure that you check the pattern before you spend the money on materials to make it, as just like in Blacksmithing, there are a ton of great recipes and patterns that you will find that will be BoP (bind on pickup), which means that you can't sell them or trade them. These are just a waste of time to make, unless you want the item for your toon.
In the next section of this guide, we'll talk about Tailoring, which is always a great profession in terms of making gold.