The first time I had knee pain when hiking was on the last few miles of a 26-mile hike to the top of Mount Haggin in Montana. I had to actually stop walking at some point. There was just too much pain to take another step. I laid down with my legs uphill for a while. That seemed to help.

That was the first trick I learned to reduce my knee pain. My particular problem is only when I am hiking downhill after putting in some miles. After some internet research, I have discovered that it may be due to "pronation" or flattening of the arch. The following are some pain-relief techniques that have worked for myself, and some that others have used successfully.

Walking Backwards To Relieve Knee Pain

I can't say that I really recommend walking backwards down mountain trails. It is a good way to fall and get seriously hurt. On the other hand, I have done this for up to ten minutes at a time, when it was a matter of getting back to the car or being stuck on a mountain unprepared. In my case, the pain relief is immediate once I start to walk backwards.

Other Ways To Relieve Knee Pain

Shoe inserts. I haven't had much of a problem with my knees since I started using orthopedic shoe inserts. I bought mine for $6 or so. The ones with the arch support are what you need, because these prevent the pronation that causes knee pain on those downhill stretches.
Knee braces. Try the neoprene ones that are sold in pharmacies. This is a cheap solution (under $10) that has helped me a lot.

Wrap your knee. If you have nothing else with you, you can try wrapping your knees with an ace bandage.You could also try wrapping your knees with a handkerchief, socks, or other clothing.
Know your limits. If you have regular knee, pay attention to how many miles of steep terrain you can hike before the pain starts. Keep this limit in mind when planning hiking trips. When you reach the halfway point, turn around.

Trekking poles. Trekking poles or a good walking stick can help take the pressure off those knees on the downhill stretches. If you have to, you can cut a dead branch or small tree to use for a walking stick. Make sure it is long enough (up to your armpits), because a longer stick works better on the downhill stretches, where you need it the most.

Cool your knees. You can try soaking your knees in the cold water of a stream or lake to reduce the swelling and pain. In winter or in high in the mountains in summer, you can use snow as well. Just wrap a handkerchief or shirt around your knee and pack it full of snow. Be careful, however - frostbite or hypothermia are real risks in winter.

There are other ways to treat knee pain when hiking. There are also ways to prevent it. For example, hiking up to your limit - but not beyond, may help strengthen the muscles around the knees, and so prevent future pain. Try the tricks listed here and see what works best for you.