Hiking downhill: easier on the knees
Pacing, poles and foot placement for the way down.

Going up is about your lungs. Coming down is about your knees and control. A few habits make the descent easier.
Shorten your stride
Smaller steps keep your weight over your feet and soften each landing.
Keep a slight bend
Land with soft knees instead of locked legs, and let your hips take some of the work.
Use poles
On steep sections, plant them slightly ahead of you to share the load with your arms.
Zigzag the steep parts
Where the trail allows it, traverse instead of going straight down.
Slow down at the end
Most slips happen when you're tired. Take a short break before the last steep section.
If your knee hurts on the way down, stop and see a professional before your next hike.