(from Northern Running Guide winter 2011/12)
An efficient running style is basically a circular motion against the ground borne out of a controlled fall through a slight forward lean. This position is so efficient because you move with the forces of gravity rather than against them. Consider a ball as it rolls; the momentum is created by the top of the ball moving forward (slight forward lean) as the bottom of the ball is pushing the ground away (feet slightly behind the body on contact with the ground). It needs a slight down hill to achieve this. On a flat, the ball needs frequent pushing, gaining and losing momentum each time.
Compare that of a runner who is upright with gravity on top or slightly in front pushing them back and the foot out in front (over striding), this becomes a stop/start motion, where the foot acts as the break in front on contact with the floor and the muscles have to contract to drive forward. Proponents of chi running, Alexander technique running, pose running and the barefoot brigade are all variations of this basic forward lean, gravity assisted position. The optimal running graphic at newtonrunning.com highlights this quite neatly.
Tip No 2. Lean very slightly forward