A great joint supplement is Knox gelatin. Its basically the same stuff you use to make jello but without any sugar or coloring. It's relatively inexpensive, you can buy it at any grocery store and you just mix one packet with a small glass of juice and quickly drink it. It helps your body create collagen which keeps ligaments and tendons pliable.
Also, what B Brown said, you have to stretch and roll out your legs. I have been a massage therapist for 13 years. 95% of the sports injuries I have helped treat could have been either prevented or minimized if the person stretched more and had been more flexible. Consistently stretching, using foam rollers and/or body works is imperative to maintaining your body's heath and ability to perform as and athlete. You can take all the fancy supplements in the world, they will not take the place of these things. It's kind of like having a car you never maintain but put premium gas in and expect it to run like a race car. Not gonna happen. Don't get me wrong, supplements will help but only if you give them the proper environment, they can only do so much.
...My 2 cents