Runner's knee — pain around the kneecap, especially when walking down stairs — is no fun at all and can really put a damper on your exercise routine. There are several things you can do to help prevent runner's knee [1] from happening, such as:

• Take time to get your body in good condition before performing activities that stress the knees.
• Try to lose weight. Being overweight puts additional stress on the knees.
• Always warm up and stretch your legs before you exercise. Remember to stretch after your workout, as well.
• Increase activity gradually. Trying to do too much before your body is conditioned for it can cause injury.
• Wear supportive running shoes that absorb shock well. Try to run on somewhat softer surfaces. Check out On the Surface of Things: Where to Run [2].
• Practice good form when running, leaning slightly forward and keeping your knees bent.

Source [3]