The most critical thing to think about is posture. Efficient runners lean forward about 6 degrees, not bent from the waist, but in a straight line from the foot up. You should feel like you are falling forward with each step. This posture puts you in position to extend your upper leg well behind the hips and also reduces braking.
Another thought is to avoid straightening the knees, both at the completion of leg recovery and at push-off.
More information about running technique is available in my book The Triathlete's Guide to Run Training and on the video Joe Frielk and I produced, Evolution Running: Run Faster with Fewer Injuries
www.EvolutionRunning.com Good luck, Ken