well one of the best things i can say is to do tuts....(when doing them pay attention to what they do and why) I'll start you off with this... beginner tut http://www.gamerenders.com/forum/index.php?showtopic=350251 and for something a little more interesting http://www.gamerenders.com/forum/index.php?showtopic=313836 keep looking for more and keep at them...once you get familiar with a lot of the techniques out there..just apply them in your own way.. though general things you should learn is blending, coloring, and depth...
work on the background and the effects on and around the render that youve used, just experiment and see what other people do in theirs.