Not a lot of people know about this method and I will admit, I don't use it that often. It's a good way to show off your skills. It makes use of some options under Layer Styles.
1. Open the image in Photoshop. Double-click in the space next to the layer name in the layers pallet. This will bring up a dialogue box.
2. In the dialogue box, make sure that "Blending options: Custom" is highlighted. Then go down to the area named "Blend If" and start adjusting the sliders under "This Layer". You will notice that dragging the white sliders will begin to hide the light areas of the image. The further you slide it, the more it will hide.
3. For even more control, hold "Ctrl" while dragging the sliders. The triangles will split which will allow you to feather out the parts of the image that are hidden.