Here is a quick trick that will make your photos look 100 times better.
1. Bring up the Levels information (Image -> Adjustment -> Levels)
2. Adjust the small triangle markers to slice off any space that has no information.
3. The triangles represent the black and white points of the image. The black triangle represents where the image is completely black and the white triangle where the image is completely white. By cutting out the part of the image where there is no information, you will heighten the brightness and contrast of the image. The middle triangle represents the mid-tones. You can slide this one too to see how it affects your image. Take some time to move the triangles and see how your picture is affected.
4. I generally run all the pictures we plan on keeping through Auto Levels (or Auto Tone in newer versions of Photoshop). This just does the steps we went through automatically. Keep in mind that some information from the original image is lost during this process.