This was posted on RT.
To run down hills effectively you need to practice running downhill. When runners do hill workouts, they usually pick a hill with a fairly good grade to it. This is great for helping to develop technique and strength. However, if the hill is somewhat steep, running down can be a little intimidating. As downhill running is quite hard on the body, choose a hill that is not very steep. In fact, a slight downhill would be best to start with. When running downhill you want to lean ever so slightly into the downhill. This lean should be initiated from the ankles, not the waist, and, again, is very slight. The key to effective downhill running is let gravity help you. Hey, we fight gravity all the time, so use it to your advantage this time. This means that you shouldn't be heel striking as you run downhill. When you run, whether you are on flats or hills, each time your heel is the first contact point with the ground, you are braking. For your heel to contact the ground first it needs to be in front of your center of gravity and due to the angle of the foot, you brake. You brake just a little, but over a marathon, it can add up. So as you run downhill, make sure that you striking the ground with the middle of your foot. This means that your foot will be underneath you rather than in front of you, and your heel will be slightly off the ground (maybe 3 centimeters/1 inch). It will feel a little odd and you will notice yourself running faster, which is why we are starting our training on a very slight downhill. You will also find that your turnover (the speed your legs are moving) will increase. This is normal, and you should work on developing a faster turnover so that you are able to take full advantage of the downhill. Practice this skill by doing a few repeats of 50-100 meters down a hill (remember, this is a very slight downhill). Focus on being tall and allowing your feet to land directly below your center of gravity. Maintain proper running form and cruise down the hill. You can try this 4 times the first time and then every couple of weeks revisit the hill. You will notice your confidence increasing, as well as your speed.When running uphill, you should again lean a little into the hill. Instead of thinking of running 90-degrees to the ground, let's think of running at 90-degrees to the centre of the earth. If you were to be running at 90-degrees to the centre of the earth on the flats, then you will want to be around 87-degrees to centre of the earth on the hills - both up and down. This is a very slight lean forward into the hill. And remember, that the lean is initiated from your ankles, not your waist. Don't bend over at the waist! Instead, stay tall and lean forward with your whole body. Your turnover will increase slightly for both the uphill and downhill, but it is not a drastic change.Hopefully these techniques will help to ameliorate the train like sounds emanating from your stride as you run downhill!
Since I tend to overstride (heel striker), instead of trying to land on my midfoot, I try to run like I am on hot coals and eggs. Of course, that means I have burned feet and broken eggs, but it seems to help a bit. Also, try to remember to breathe going downhill- it'll help you relax.