I agree with the weight lifting. Get a personal trainer consultation at your local gym (preferably a cyclist and/or familiar with knee problems), and have them set up a routine that addresses your knees. If you build the muscles around the knee, it can help stabilize them (I've never had knee problems, but this is the theory). You can also develop power through plyometrics(?) which is things such as step ups and jumping. A trainer will show you proper technique.
I hesitate to recommend yoga DVD's because everyone is correct--if you have "issues" you need a good yoga instructor who can build a program for you with modifications. However, personally I have lower back issues and I like Rodney Yee's "Yoga for back pain" and "Sports conditioning--mini yoga workouts." I've been meaning to post this mini cycling workout here, so for those already familiar with the poses, here it goes:
(purpose is to undo the forward bending of cycling and open the chest and shoulders)
backbend
downdog
left lunge
downdog
backbend
downdog
right lunge
downdog
right side angle standing
downdog
left side angle standing
downdog
cobra (twice)
hamstring stretch on your back w/ strap
Put a rolled up blanket under your shoulder blades--arms overhead