Using trekking poles can reduce the impact your bodyweight (+backpack weight) when hiking downhill. It can also be helpful to do exercises that target your quadriceps and hamstrings. Exercises like biking, squatting, lunges, and step-ups should help. These are all bodyweight exercises and don’t require the use of weights.
If you find that the pain is unbearable, and you visit a doctor, be skeptical if you have knee pain and your doctor recommends surgery. Physical therapy can provide you with an alternative cure and help you understand how prevent a recurrence. Be sure that your taking care of your body - it really is the best support for an active lifestyle.
If you experience pain on the trail after a hard day of walking, bring along some ibuprofen to reduce the inflammation at night and stretch your quadriceps before sleep (short term). For a more long term fix, try supplementing with Fish Oil regularly. It has anti inflammatory effects on your body and can really help with joint pain (also check into Glucosamine and Chondroitin as a natural remedy). You may also find that wearing a knee brace is helpful. Cho-Pat knee patches can be worn to alleviate stress on the patella tendon.
Here's the take home...
1) Make sure your shoes fit and have good soles. Replace hiking shoes every 400 miles or so (if knee pain is a severe problem for you
2) Use trekking poles to help support your body weight
3) Exercise regularly
4) Ibuprofen, fish oil, and glucosamine and chondroitin can help
5) Try a knee brace or cho-pat strap
Please note, we're not doctors. This is not professional medical advice.