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 animation.
A while ago at the NYC Blender Conference (website offline) I saw a great presentation by the then teen-aged talented modeler Jonathan Williamson, who can build a head from scratch using this technique within the hour. He is one of the founders of cgCookie.
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.
Here is a timelapse video from YouTube, using Blender :