The problem:
When running downhill, some people have a tendency to lean way too far forward, overstride, and run out of control. Running downhills improperly like that can lead to injuries.
The solution: The best way to run downhill is to lean forward slightly and take short, quick strides. Don't lean back and try to brake yourself. Try to keep your shoulders just slightly in front of you and your hips under you. Although it's tempting to overstride, avoid taking huge leaping steps to reduce the pounding on your legs.

More: How to Run Hills
Benefits of Hill Running
How to Do Hill Repeats