Via Jan Kabili:
Take a look at your latest batch of digital photos. Chances are the biggest problem with most of them is lack of contrast. They look dull or muddy because they don’t have the full tonal range of black blacks, white whites, and lots of gray shades in between. That’s easy to fix in Photoshop with a Levels adjustment (pull in the black and white Input slider to the edges of the mound of data in the histogram) or a Curves adjustment (drag the curve into a classic, subtle S shape). But sometimes too much contrast, rather than lack of contrast is the problem with a photo. Can Photoshop help you there? Ben Long offers a cogent explanation of the problem and some practical solutions (like the Shadow/Highlight adjustment) in this article at Creativepro.