1.) Make sure you have a program capible (Texture Maker, Photoshop, ECT.)
2.) Make sure you have a decent computer that can process functions well (depends on how much memory the program eats)
3.) Have good knowlege on the program that you are using (practice makes perfect)
4.) Make sure you are good at making textures (Crappy texture design will not sell, obviously)
5.) Make sure you get the job done on time. If you can't make the deadline then tell the person your reason well before hand.
6.) Time and dedication
These are the must-have requirments on texture making even work in general.