Skinning and mining are no doubt the best professions when you start out. But after that a profession like blaksmith is a good one to replace one of them. Resources to start this professions are quite expensive and initially you will be some gold down, but after that clearing in excess of 20 g a day should be a breeze.

In Tanaris there is good gold to be made killing watewander bandits and thieves which are level 40 and 41. Other mobs here range between 42 to 45 and have some nice drops which are worth collecting. Here the Glasshide Basalisks are also a great find and lurk all over the place in Gadgetzan. They are red or blue colored and are great money maker in world of warcraft at these levels as you will find. Their tails and their eyeballs yield at around 7 silver and the prismatic scales can fetch up to 14 silver, as well as one gold piece on the few occasions.

Try this area yourself if you have reached lvl 40 and are in need of gold, especially if you do not have a mount yet. This could be the place to get it.