Run Downhill

Old CW: Run up hills, then jog back down.
New CW: Run down hills, then jog back up.

Downhill running is integral to the training of elite distance runners. Running downhill increases stress on your legs, which makes them better able to handle impact—as long as you don't overdo it. Running downhill can help you go faster, because your muscles will grow accustomed to the quicker stride required.

Once or twice a week, after completing an easy run, do four to six relaxed downhill sprints (not on a steep hill, just an easy grade) lasting about 20 seconds apiece. Recover between sprints by jogging slowly back up the hill.

http://www.menshealth.com/mhlists/running_efficiently/Run_Downhill.php