I found that doing some light weight/resistance training at the gym strengthened tendons and such around my knees and helped my performance.

Some like to take Glucosamine/Chondroitin but the jury is still out there for me to form an opinion.

Don't forget the standards like good stretching, cross training (I also cannot run more than two days in a row - must also do low impact aerobic stuff like cycling), and proper shoe fit. I added arch supports to my shoes this spring and was amazed at the difference in my recovery post workout and race.