When I shade, I try to follow normal contours of the body and clothing. I don't like overshading, so my shading seems to lend a cartoony style. For example, your back slopes inward, toward the front of your body, unless you have a hunchback. In this case, the shading would go from light near the shoulders to dark near the waist. For clothing, ridges are lighter and valleys are darker. I tend not to overshade the skin, using only one or two colors to shade the face and bare arms.