Pushing the sled appears pretty straight-forward. However, to push the sled efficiently, you need proper body mechanics.
Since the sled is moving horizontally, you need to direct the majority of your force in that direction (orange vector), and minimize up and down forces (yellow vector). As you can see from the picture, your driving force (purple vector) needs to be at a low angle for this to happen.
You also need to stay at this low angle to to effectively use your hips to drive the sled. Notice the full hip extension. They should be the main contributor to your sled drive.
Remember these cues:
• keep torso angle <45 deg
•drive heel back and opposite knee forward.
• long orange vectors!