I agree with hannad about the books. Besides that, you could take a look at the advanced tutorials of cprogramming.com: http://www.cprogramming.com/advtutorial.html. If you don't know them already, you could learn how to use graphics: opengl, allegro, SDL, ...
What you might want to do is decide on a project that interests you and work on it. As you work on it you'll find gaps in your knowledge and you can fill them in.