OK, I'm a nerd...Many years ago this question cam up and Matt Carpenter replied with good advice. I saved it...Here it is for an Encore:
...Actual technique is almost another thread but the underlying principle is to lean forward with your whole body so that you are still at least perpendicular to the ground. Otherwise, while you may be upright, because of the downhill you are really leaning backwards which contributes to the problem. Next, your foot must land under you and not in front of you so that there is no braking. The biggest contributing problem here is people tend to over stride when running downhill. A major symptom of this is hitting hard on your heals. Your foot should not stick or jam but should already be in motion backwards when it lands just as what should happened when running on the flats. This concept can be practiced by standing on the side of a treadmill. Just drop one foot onto the treadmill and you can feel how your foot is jerked back and if it is going rather fast this can even hurt. Now try again with your foot already moving at the speed of the belt as you place it on the belt. No issue at all. Many, when running downhill, are basically doing the equivalent of dropping their foot onto a moving treadmill. Is it any wonder so many end up with those really painfull looking silver dollar-sized heal blisters? Well for some they are not even blisters and qualify as open wounds as the skin has been ripped off. OUCH!!!