Just think of a real face when shading your own. Remember to put darker colors in the spaces that have shadows cast onto them, or the objects that don't face the light quite as much. Add lighter colors to things that are more exposed to light, and objects that cast their own shadows onto others.
I hope that helps. And here is a bit of advice. Don't EVER use the Light/Dark (In your case, Dodge/Burn) tools. Ever. They make you get too used to using their patchy method of shading, while keeping you from improving on the real thing. Okay?