You generally create a new layer, and fill it with a nice skin color. You may add greater detail to this area if you need it. This provides a background for you to arrange the photographs about the uv map so the nose in the photograph covers the nose on your uv map. It involves using the use of layers, slpicing, and blending until it renders without error. You usually have poser and photoshop open so you can make frequent tests. You need to remove the shadow from areas like the chin, etc. After making a few you'll get the hang of it. I hope someone with a greater grasp stops by to explain.

One way is to start with a  baked normals map, and a map of the wire mesh. The baked normals map with the wire mesh map as an overlay provides a nice way of painting a color map, or merging several photographs as a color map, using skin resources and a painting application, etc. It better shows you where the areas you are texturing lie, which means fewer trips from photoshop, Z-brush, etc to the application you use to render your model. I like to create the displacement, or bump map first in Z-brush and photoshop, and use it in concert with the normal and wire mesh maps.
This is how I do it anyway, but be aware I'm no expert.

Robert Undi's method is a better, more difficult way, and may be found in 3D Artist issues 41-43. Essentially you create a turntable for your camera, place a chair for the model to sit on at its center, and take fifteen photographs from position 1 at 0 degrees (the front view) to position 15 at 90 degrees (the side view). You may do this with the model standing up too, add more frames until you have gone a full 360 degrees around the model.
In photoshop load your 3d models template, and the photographs; place the photographs so  that they overlap each other. After removing shadows in certain areas, and blending the photographs where their edges overlap/meet in photshop you achieve an end result, which looks as though you unwrapped a human. Now you can adjust the map using something like surface suite pro in 3ds max, or photoshop so that it fits your 3d models uv map properly.

The example of an unwrapped baked normals map is from a 3d.sk tutorial.