Photo Reference Help:
5 Crucial Photoshop Tools for Photography
Tutorials for Photo Retouching
Collection of Photoshop Actions
While we are on the subject, remember that the best practice is to use photographs you take yourself so that you do not have to be aware of copyright infringement. HERE is a great article from the American Society of Portrait Painters about copyright law.
You should also take LOTS of photos of your subject-think about having images that are over-exposed and under-exposed so you have information for the lights and shadows. Make sure you are using a clear, directional light source and that it is placed in a way that creates an even play of values across the form.
James Gurney's blog is an excellent resource for portrait lighting tips:
HERE is a great article by Rusty Jones on How to Use Good Reference Photos.
Also check out my previous post about Norman Rockwell and other artists' use of photo reference.