Personal trainer Matt Hart replies: A tight Achilles tendon/calf muscle can cause you to rise up onto your toes when you squat, knocking you off balance. If you think this is the problem, try placing a thick weight plate or piece of wood under your heels as this will take the pressure off the area. If this isn't your problem, have a look at the following technique guide and try again: Stand to attention with a straight back and your knees very slightly bent (soft). Rest a bar comfortably over the top of your shoulders (use a towel/pad for comfort). Slowly lower yourself until your knees are at 90, with your back straight, tilting forwards from the hips, head up and your backside pushed out. Slowly push up through your heels to the start.
WRONG! Bar resting on neck stresses the cervical spine. Rounded back stresses the lower back. Weight too far forwards takes the load off the legs and places it on the back.