Actually.... The best way to improve you knees totally depends on what is "wrong" with your knees. The exercises Fitzy suggests are great if the joints are in good shape. If you have the kinds of problems that I have, those same exercises would damage my knees so badly I couldn't walk. The best thing I've found for *me* is the right stretching and lots of cardio that doesn't stress my knees at all. Try a variety of things, and learn to listen to your body. It has a lot of wisdom. You only learn how to hear it.
If its the joint, try laying down, bending one leg and raise the other straight and tilt it in a bit, count to ten and switch legs. you're supposed to do this a few times every day to get strength and work it in.
I have bad knees. Especially my right one, that I broke and all sorts of shit happened to it, besides my knees are fucked-up like they're inward or.. I don't know how you'd say it.. like not straight, they go towards the inside of my legs. I already have athrosis or whatever it's called too, at 23, so imagine when I'm 50.. meh I'll get new ones.
I avoid kneeling or getting onto my knees because they click everytime I do. I'm pretty sure that they'll be causing trouble for me in the future.