Box Squat

Set a box or the end of a bench where your rear end would normally end on a squat. It should be about 14 to 18 inches high. Now set up for a normal squat.  
 
Push your hips back and lower yourself until your rear end touches the box or bench. Don't stop the movement (absolutely don't relax your lower back); rise back up as you would in a normal squat. The box is only there to help you learn to use your hips, instead of your knees or lower back, as the prime movers in the squat, and to help you squat to a lower depth, if that's an issue.