Contrast Adjustments

I recommend trying the standard AUTO adjustments first because they often do a good job at quickly improving your images:

Enhance>Auto Contrast This will automatically improve the contrast of the image. This is a great way to improve the contrast of your image without affecting the color

Enhance>Auto Color Correction This will help improve your color automatically

Enhance>Adjust Color>Remove Color Cast With this tool, you use your cursor (eyedropper) to click on some part of your image that should be grey.  This will help you re-configure the colors of you image.

Color balance is slightly too green Adding magenta helped bring the color into balance

Enhance>Adjust Lighting>Levels

The levels feature is a great way to allow photoshop to make the adjustment of your photo's contrast an easy task.  The levels is a statistical graph showing how many pixels are dark and light and somewhere in the middle.  You can adjust the black, grey, and white arrows to adjust the contrast

The screen in the levels dialog box with the funny black curves is the histogram of your image, or in other words, a visual representation of the tonality values of your image.  Just below the histogram, there are three arrows: black, grey, and white, which correspond to the shadows, midtones, and highlights of your image.  By adjusting grey arrow, you can adjust the brightness or darkness of just the midtones in your image--while leaving the blacks and whites alone. This is a good technique, because in many images, you don't want to compromise the shadow and hightlight values, just the midrange values. Notice in the images below, that the very bright tones and the very dark tones remain relatively the same.

Original Dark midtones Light midtones