The biggest trap is pushing with your arms. Your legs are your engine here; your arms are just steering. Get low into a partial squat, hands on the sled at about shoulder height, and drive your legs into the ground. Each step forward is a push from your legs that moves the sled, not an arm press. Keep your torso upright and your core braced. When you start to lean forward and reach with your arms, you've lost the position and turned it into a press instead of a leg drive. Initiate every rep with your legs.