A method of running called Chi is becoming popular. Two attributes of running-form procribed by Chi are mid-foot strike and leaning forward a bit from the ankles. I'm automatically changing from a heel striker to a mid-foot striker because I run a fast stride rate of about 180 steps per minute. A faster stride rate means I take smaller steps, and (for me at least) smaller steps mean I hit the ground with my mid-foot instead of with my heel.
A few days ago I was reading about Chi, and I read that I should lean slightly forward (from the ankles not the waist) so that gravity will pull me forward. I've been trying that, and it really works. I run faster and with less effort due to the effect of gravity on my body. By leaning forward slightly, my center of gravity is slightly in front of my feet, and that causes gravity to pull me forward instead of just pulling me down. It doesn't take much of a lean to get the benefit of gravity. In fact, if I lean forward too much, I'll fall down. The risk of falling down is even greater when I'm going down a hill. By taking smaller steps, due to a fast stride rate, my foot hits the ground when the leg is moving backward, and this means my leg isn't doing all of the work to propel me. It is just lightly touches the ground to keep me moving.