Luminance Masking - Page 7
Duplicate the Background layer by dragging it to the "Create a New Layer" icon next the to Trashcan in the Layer palette. Rename this layer "Highlights" by clicking on the "Background copy" title. Click on the "Add Layer Mask" icon. It is located at the bottom of the Layers palette, second from the left. Change the blending mode to "Multiply".
Your image should now look like this:
Click to see the "before" version.