I was having knee pain about 2 years ago when I was training for the MCM in 2011. I ended up not doing it and defering to last year. Anyway, I went to the Dr because I was concerned that it was going to keep me from doing the race. They did x rays of my knee just to make sure there wasnt anything broken or something. The PT diagnosed me with "tracking patella". He said it was due to my being overweight and having weak hips. I was to work on strengthening my hips and thighs and it would in turn help with my knee issues. He said it was common in women - as it's common for women to have weak hips.
Anyway, I did not work on strength training and just bought this:
Which it does help. I rarely get pain with it anymore. But I want to eventually be able to stop using it and have been trying to work on what the PT wanted me to.
It was interesting after this happened, I started reading a ton of articles in exercise magazines about this exact thing and how it stems from weak upper leg muscles.