Two simple rules to start with....
A) your render should mean something, say something, stand for something.  merely twisting dials and adjusting the lighting on your new Vicky may be enough for some people, but your work has to be more than pretty.  It has to have a story, meaning, depth.
B)  your render should reflect your character, your thoughts, your dreams, your desires.  this is, after all, your work, your art.
Having said this, I'm going to get into trouble with people who want you to concentrate on the technical aspects of the program, however (and there is always a however) you should use these tools as a story telling medium.