You don't get to my age without having a few aches and pains and in my case it's my knees that are the weakest link. However I've found a few things that help.

First of all I take two 400mg Glucosamine Sulphate tablets every day. The tablets that I use also contain Chondroitin Sulphate, MSM (MethylSulfonylMethane) and Vitamin C.

Secondly I try to find exercise methods that don't stress them too much. I used to do my cardio on a rowing machine, but I find the action of elliptical trainers seems to be much more kindly to them. I've also adapted the way in which I use the ellipticals. I'm slightly knocked-kneed by nature so to compensate for this I stand with my feet slightly splayed. I use an action which gives me the sensation that my knees are going outside of the vertical line, but when I look down they are moving pretty much straight up and down.

Finally I have re-introduced deadlifts and stiff-leg deadlifts back into my weight training regime, with the hope that having more strength in my quads and hams will be good for the knees. I'm doing this progressively, being quite careful with the weights. So far, touch wood, I seem to be able to do this without causing any knee pain.