I bought a couple of books from this guy, Michael D. Mattesi,
Force: Dynamic Life Drawing for Animators and
Force: Character Design from Life Drawing, both are really interesting and insightful whether or not you are into animation and have tons of examples showing what he is talking about.
There is a bit I want to quote from the Character Design book because it is really great advice:
Fear: Here I am again writing about fear. I have been teaching an additional six years since the first printing of FORCE and I can honestly say again, that fear is the biggest detriment to the forward pursuit of education. Fear comes in all forms, some more obvious than others. The fastest way out is the listen to your internal dialogue. Notice when and why you are indecisive or concerned. Let drawing be about your experience and curiosity, not the final product. You create the fear to rid yourself of it! It will only slow you down.
Risk: Well, we just spoke about fear and here we are talking about risk. Doesn't that cause fear? Yes, good fear. What can happen to you if you take a risk and you don't get the results you want? In order to have opinion, you MUST be able to take risk! You MUST fight your fears. You MUST be willing to fall on your face to pursue you creativity! Once you break the bonds of fear, and love feeling risk while you work, you will never turn back. Besides, we are not sky diving, we are drawing. Nothing can hard you!