Exercises that strengthen your thigh and calf muscles will strengthen the muscles that support your knees. If you haven't been doing these, start slowly and work up gradually. The idea is to strengthen the muscles rather than aggravate anything in that extremely complex joint! I tore nearly all the ligaments in my left knee 23 years ago, requiring extensive surgery, and the daily exercises (when I'm home) are what keep it going. I do wear a lightweight neoprene brace on that knee for long downhills, but don't unless your physician recommends it.

Trekking poles will help. Here is a good site on how to use them. So will lightening your load--see the articles listed in the left-hand column on the home page of this site.

But please don't walk backwards, and stay on the trail. Please!