When starting a face I first try to find the areas of highlight (lights) and shadow (darks) that define the plane changes on the face. It's tempting to go into great detail on one facial feature, like an eye, but it's better in the end to make sure all the general areas are correct first. I've made the mistake before of perfecting an eye and then realizing I painted it too high or too low or too far to the right or left and having to repaint it. Lesson learned...