View this tutorial and others on Blender Cookie: cgcookie.com/blender/2012/06/13/creating-realistic-head-blender/
This is a complete tutorial series explaining the creation of a realistic human portrait with Blender. The entire process will be covered from base mesh modeling, detail sculpting, texture painting, hair growing and styling, sub-surface scatter shading, and compositing. Some of the more time-consuming tasks will be time-lapses with commentary like modeling, sculpting, and texture painting; the other parts will be mostly real-time.
Part 1 explains a more artistic approach to generating a base mesh instead of traditional box modeling techniques. It’s a hybrid method incorporating both modeling and sculpting. We’ll start by sculpting our basic forms and volumes, then retopologizing a usable facial topology that will serve as an excellent base for sculpting our high-frequency detail.