Another way to do it:
The most important thing is that the source photographs are
suitable ( position of the head, light sorce and etc.)
1. crop the head from the photo and drag into body photo
2. add a layer musk to head and hide everything,except the face
3. If needed use transform tool to rotate or resize the head
4. Add adjustment layer levels to head and group with previous,and new face will match colors of the body
5. use airbrush on layer mask to blend the face.
The tutorial that I learned from you can find here:
http://homepages.ihug.com.au/~larryk.../contents.html
> Go to FAKE FACES.
Hope this helps.