Contour Lines

Adding volume to drawings.

A polaroid pistol A mannequin torso A doorknob A ball
A polaroid pistol, mannequin torso, doorknob, and ball

You can quickly add volume to an object by using Contour Lines.

These are simple lines that may convey information like the curvature, orientation, and depth of an object.

It's a simple technique, but it completely changed the way I visualise and draw things.