The Luminous Landscape has a tutorial on Local Contrast Enhancement Using the Unsharp Mask.  The driving force is the the standard Photoshop Contrast adjustment is often not effective in dark areas because it often makes all of the dark areas darker.  Their web site gives additional details, so this page will be brief.  The following technique is effective in many situations, but beware of the potential artifacts listed later.  The Unsharp Mask is primarily used to sharpen images, and as noted in my page on Sharpenning, the Unsharp Mask is applied directly to a Pixel layer, thus it is not an adjustment layer that can be modified after the fact.  Because of this, you might consider duplicating the pixel layer and/or duplicating the original image file and work on a separate new file.

For general sharpening using the Unsharp Mask, the Amount is often in the 100 to 300 range, and the Radius is very small, about 0.6 to 1.0 of a pixel.  For Local contrast, the Radius is very large, about 50 or 60, while the Amount is relatively small, about 20 to 40.  To get to the Unsharp dialog box in Fig. 1, go to Filter > Sharpen > Unsharp Mask.
 

Fig. 1  Unsharp dialog

Poential artifacts:

• If the image is "noisy," the digital noise will become worse.
• You can get a large "halo" in a situation where a blue sky meets a dark mountain range.  Since such a situation does not usually need additional local contrast, either apply this technique to a selection that excludes this region, or erase the bad areas if you duplicated the pixel layer.
• Bright highligts can get washed out, so either exclude these in a selection or erase the bad areas if you duplicated the pixel layer.
• As with Curves and Contrast Layer Adjustments, there will likely be a color shift in Normal Blending mode.  Often, this color shift is desirable in that richer colors result.  With Adjustment Layers, you can easily change the Blending Mode to Luminosity if you do not want a color shift.  Since the Unsharp Mask is applied directly to the Pixels, you can change to a Luminosity Blending Mode by the following technique.  You can eliminate potential color shift artifacts by Edit > Fade Unsharp Mask ... immediately after the Unsharp Mask has finished.  When that dialog box appears, click on the Mode drop down menu and change the Mode from Normal to Luminosity (which is at the bottom).  I suggest toggling the Preview on and off to see if you want the Luminosity or Normal blend.  If you like the original (unchecked Preview), simply click Cancel.  If you like the Luminosity blend (checked Preview), click OK.