Look for a photo of a head that features a strong shadow pattern. Imagine it as a series of layers: First, an overall pale wash to represent the illuminated skin tone, into which color variations of similar value can be placed. Then, a shadow pattern, which can also be given soft edged variations, and, finally, the few darkest darks, like pupils and nostrils.
If you prefer, try working in monochrome, so you won't be distracted by color. Remember to choose a single color rather than a mixed one, and make sure it's a color that can get dark enough to represent the deepest darks.