Character Artist at Blizzard Entertainment, Tyson Murphy, did a neat little exercise where he took a scene from a movie and painted over it and he ended up with a really awesome result. Give it a shot.
There is a blog http://www.floobynooby.com/ICG/artvalues.html that features everything that you can possibly imagine about illustrating in Photoshop. Just click on the site, and your mouth will drop at all the amazing things that are on there. Thank you Melissa McIndoe for this great find!