Color theory. One of the most common questions asked in this forum is "how do I get this look?" Which is usually in regards to the images color palette. If you know some color theory and how colors work in Photoshop, you should be well prepared to get any look you want.
Other than that, practice. It's one thing to know of a particular technique, it's a whole other thing to be any good at using it. The more practiced you are, the more proficient you are.