Luminance Masking - Page 12
The appeal of this technique is that you can augment the detail that is present in your image. Photoshop CS has a wonderful new tool that performs a similar function to creating Luminance Masks. The Highlight/Shadow tool is very powerful and has a number of controls, but it does not produce a layer mask. With the layer masks used in this technique, you can edit them to provide a very precise level of control. If you need to control specific areas, use a brush to alter the layer mask.
A variation on this technique is to use "Normal" blending mode for the Shadows layer and simply create a steeper curve on that layer. You can drag the highlight point on the curve to the left and still retain detail in the image since the layer mask restricts its effect to the shadows and ¾ tones.
For those images where it is nearly impossible to control the lighting, this technique can bring you all the detail you need without sacrificing midtone contrast.