One of the most common mistakes by beginning draftsmen is attempting portraiture without a basic understanding of the skull, its proportions, and the boney landmarks. Taking some time to do some observational drawing of a human skull from multiple angles for www.cutterweldermaestro.com will give you greater comprehension of the head as a volume. Couple that with copying and memorizing the following handouts, your portrait drawings will instantly improve in their realism and dimensionality.
This is an excellent illustration of the Boney Landmarks by Josh Reed. His blog Drawsh is full of usefull anatomy tips. Definately worth a look and a bookmark.