It just so happens that I'm working on a file that requires this technique. There are a couple of things that I'm doing to get a flat photo to "pop". The first thing you have to do is cut it out from your background. Once that's done then I've added through Photoshop's Layer Styles (bottom of Layer palette)...
• Drop Shadow
• Inner Glow (Black set to Multiply. This will give your edges more contour. The shadowing effect helps to make them look like they are turning away from the viewer.)
• Bevel and Emboss (Yes, you can bevel and emboss photos!)
• Sharpening (Unsharp Mask with a low amount and high radius 32/32/0 can help to enhance the larger shapes in an image.)
And here's what that looks like (original on the left, popped on the right)...