Shoot from higher above the subject if you can (especially for single subjects). Things farther away from the camera appear smaller, so when shot from a higher angle, the body becomes smaller relative to the head. If shooting more straight-on, never let the subject stand broad-shouldered to the camera (go with a 45-degree angle).

Another trick is to position such subjects behind other subjects so that their face/shoulders are the only things visible. You have to be careful with this strategy, however, as a subject will notice if you repeatedly ask them to stand behind everyone.