I would do some more studies on human anatomy. It's a tough thing to get down, but once you do, it'll help everything you draw, even when drawing in the simpler, cartoon-ish styles. It's something you wouldn't think of, but once you have a decent knowledge of the structure of the human body, that knowledge will come through, no matter what style you draw in.
If it helps, look up some tutorials on drawing anatomy, as well. Other people may explain things in ways you would not have thought of on your own.