Try to distill the character of your subject. Understand how he moves, thinks, acts. It's difficult to put into words. Consider each drawing as a problem that did not exist before, and then try to solve that problem to the best of your ability. That i what caricature is all about
You know, it's no accident that the great painters came from areas like Europe where there is a lot of clouds and rain, which begets color and subtle washes of tone. Most great graphic artists come from areas with prevalent sun, where line and shadow are paramount.