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.
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