I've had knee pain on and off for a lot of years now. I messed up my knee multiple times running track in high school years ago. It was never serious enough to have surgery and like you X-rays, etc. showed nothing.
I have prescription strength naproxen and this helps reduce the pain and swelling and is a good temporary fix.
What I've found that works best is weight lifting to build the muscles up around the knee. Leg extensions are probably the best but I also do leg presses and leg curls to build up all the muscles in my legs.
If I was more dedicated to my physical fitness I'd go to the gym every week and keep this up. The reality is I'll join for a year and go 1-2 times a week for 6-12 months and then take a couple years off until my knee really bothers me again. At that point I'll join a gym again for a year, lift weights, knee feels great, stop going to the gym, rinse, repeat.
