Poly Modeling
Here are some notes on poly modeling (as opposed to box modeling, where you start with a basic shape and extrude / refine from there)
In poly modeling we start from a single polygon and extrude edges to create adjacent polygons. It is always a good idea to create loops of polygons, around the eyes and mouth for instance. This will not only enable you to make good use of selecting edge loops, but will also give good results when deforming the shape in amimation.
A while ago at the NYC Blender Conference (website offline) I saw a great presentation by the then teenaged talented modeler Jonathan Williamson, who can build a head from scratch using this technique within the hour. Here is a timlapse video from YouTube:
One thing he stressed was: Know your anatomy! Not that you have to know the names of all the muscles, but know where the muscles live.
Of course he works in Blender (he is currently with CGcookie) but this approach is not software specific.
That same Jonathan also created this great tutorial: Character Head Modeling in Blender