Last year photographer Jimmy McIntyre published this video about how to use a tool called Luminosity Masks in Photoshop to improve the exposure of your images.
Luminosity masks can be a very powerful tool in Photoshop. McIntyre has a great explanation about the nature of Luminosity masks (also known as luminance masks) in a post he published on 500px last year) which you are more than welcome to read here. The basic idea however is this: Luminosity masks are advanced selections in Photoshop which are based on luminosity values. So to use McIntyre’s own example: If you shoot a sunset, we will typically expose for the foreground so rather typically the brightest part of the sky is over-exposed. If we were smart enough, however, to take a second, darker exposure, this time exposing for the sunset, than all we need to do, in theory, is smoothly blend the area of the sun into the brighter exposure where that same area is blown out.
There are many ways to achieve this, however using luminosity masks you can do this rather easily by selecting the over-exposed area because of its luminosity values and smoothly blend in the darker exposure into a well balanced image.
McIntyre has a set of free luminosity mask actions which you can download from this link if you want to try them out.
You can find many more Photoshop video tutorials on LensVid’s Photoshop section.