shadows

Start by getting a clear 2 value system. The edges of the value can be soft firm or hard. After that, you can add half tone, core shadows, and dark accents. The key to making your drawings look solid is to have the shadow pattern relate to the contour. Sometimes you can't even see the change in the contour, but because you see the change in the shadow patter, you know it has to be there, so you look for it and find something you didn't see before.