You need to make some important calculations to show form. First, find the planes of the brightest brights. Next, find the planes of the terminator. Next, find the halfway point between the plane of the brightest bright and the plane that faces the camera. Once those planes are there, just measure the speed at which transitions happen between them.
Render form shape by shape. Get the big mass in first, then medium, then small. Start with a flat value (preferably the darkest), then add the brightest bright, then highlight if needed. Oh yeah, don't forget the cast shadow, too! Add ambient occlusion and reflected light, and there you have it!
Scott Robertson says: "form change equals value change" so choose which forms you will render wisely.