You should start at my latest Squidoo article, A Comprehensive Guide To Drawing the Head, which outlines all different variations of drawing the head, including turns, expressions, head tilts, a number of resources, and Preston Blair's phonemes series.
So, there you have it. But, if you just want the quick and dirty. Here are all the images I used.
Turning A Head
|Male Head Guidelines|
|Male Head Features|
|Male Head Final|
|Female Head Guidelines|
|Female Head Features|
|Female Head Final|
Preston Blair's Phonemes Series
|Preston Blair's Phonemes Series|