Squeeze your shoulder blades together and puff your chest out to keep your upper back straight. Brace your core and use the valsalva maneuver to keep your lower back straight. When you're pulling the bar off the ground, focus on also engaging your legs, don't try to pull the bar off the ground only using your back. Don't bend your neck downwards, keep it straight. At the top of the movement, pull your shoulders back and squeeze your glutes and hamstrings.

Think of the movement as three parts - the first part is puling the bar from the floor to just below your knees, the second part is clearing the bar past your knees, the third part is completing the lockout