Here is a link to a short video tutorial showing one way to add folds & creases to a map (or any image, really) in Photoshop. It is by Corey Barker, "Executive Producer of PlanetPhotoshop.com and an Education and Curriculum Developer for the National Association of Photoshop Professionals."

http://www.planetphotoshop.com/folds-and-creases.html

Thanks to Butch for posting this link on his website where hungry learners like me can find it!