This might not be the best way, but it's how I do it. It works well with characters.

First, make sure you have a clean material assigned to your object. Select your object and choose Rendering Menu>Texturing>3D Paint Tool options box. In here, assign your object a texture. 512x512 is good. This assigns a file texture to your object's material - hence why I wanted you to assign a clean material to your object otherwise the file texture would have been assigned to the default lambert1. Now you can paint on your object. Usually, I use common colors like red, green, and blue to paint arround the lips, eyes, nose, etc defining the position of everything. This can be done with any object of course.

After you have drawn a little on the object, choose Save Textures in the 3D Paint tool options window. This saves the texture you just painted as an image file. Now, open Photoshop, locate that particular image, and do whatever you want with it. Since you painted around specific features on your model, you know exactly where these features' texture spots are on the image file which makes it easy to modify them.

Where you go from there is up to you. I once took a front shot of my face, modified it a bit, and stuck it on the image file. Since I had the nose, eyes, and ear spots identified on the image file already, it was extremely easy to align my facial shot to where it should have been.

Common problems - If you can't paint on your object, you need to remap your uv's. UV's must be in the zero to one range for you to be able to paint.

Also, once you have your image the way you want in Photoshop, all you have to do is save it, then load that image file onto your object. Be careful with targas though as a transparency thing will be automatically mapped to your material. Just right-click "transparency" and choose "break connection" if this happens.

Hope this helps some.