Please notice that it will certainly take several approaches and a lot of time and patience until you end up with a more or less realistic looking human head... I think I modelled at least 20-25 heads until I ended up with something I call a "basic head". That is a very neutral, very undetailed but correctly modelled head with a clean edgeflow. Starting with this basic head I can easily and quickly design new heads and shapes by simple doing some poly-pushing in soft-selection mode. The mouth and the eyes can also be opened using morph-targets.
Personal hint: I really really recommend to do the same thing.. as soon as you think the basic proportions are blocked out and the whole thing looks nice when smoothed, save your head as a unique model or scene, so you can go back to this stage whenever you want to. It might save you a lot of time...
The following pictures show a rough working-progress on the actual picture. The most important steps will be explained afterwards:
Here is a wireframe of the finished model. You might find it helpful to see before reading the making-of: