The best way to create the illusion of mass and form is pushing the value range to its limit. Bright highlights and dark darks are pivotal, regardless of your chosen medium, the subjects you work with, or the style in which you render them.

This is the subject of my latest EmptyEasel article, which you can read here. How to Get the Most Out of Your Colored Pencil Values describes how to improve contrast by pushing the value range as far as it will go.