A great way to learn to draw human portraits is to draw human skulls, as the human skull is the foundation of the form and shape of the human head. A great way to get better at drawing human skulls and other organic forms is to draw animal skulls. This site is a great collection of photos of a huge variety of animal skulls. The photos are really clear and nicely done and great for reference drawing or practice.
Also, linked from Will's Skull page is Jan Jansen and Wouter van Gestel's Bird Skull collection. Their site is an equally lovely collection of bird skulls from a huge variety of species.