Adapted from my written tutorial at bendansie.com/
NOTE - When in Gimp/PS matching the photo to the rendered angle, turn the rendered angle layer OFF before projecting and have a plain background for the rest of the image. That way it will be easier to blend the projections together. Forgot that myself.
NOTE 2 - (Shouldn't have made this at 11pm...) The plane is 1 blender unit from the head, not camera, but being orthographic it only really matters that the plane is behind the head for rendering. Keeping it all to 1 does keep it simple though as stated.
NOTE 3 - It was suggested to me to merge the layers using a layer mask - that way it is non destructive blending between the projections.
Due to request, it's now a video tutorial. It's my first video tutorial, so it isn't perfect, but I'll be revisiting this topic later (most likely) when working on a human character documentation project with some help from other BlenderArtists.org artists. More to come on that when I get the time.
Used Blender and Gimp only, so if you have a model and photos, this can be done for free (excluding the price of a computer to do it on of course...)