Tutorial: http://www.metacafe.com/watch/1663770/x_ray_your_friends/
Examples: (contain nudity, obviously)
http://www.metacafe.com/watch/1663793/amazing_x_ray_technique_demonstration/
http://www.metacafe.com/watch/1664196/g4s_olivia_munn_x_rayed/
If anyone's tried this, I could sure use some tips on how to match the skin tone better

Everything else I can do rather well.