What winterclaw said -- and when you've got that done, to go to the next level involves color theory. You should have a predominant color throughout the image (as in real life) and try to reach the entire dynamic range to (try to) imitate the true range of real light. ANd resolve yourself that you will always be a student of Poser - there are no masters.

To try an illustrate what I mean by dominate color, her'e one of my renders using a green as dominant:

http://fc00.deviantart.net/fs70/i/2012/308/b/b/the_beast_of_cloister_hall_by_tonyandrulis-d5jyh8d.png