Couple things you can do -
- put small plates under your heels
- sit way back into your squat - make sure your butt is sticking out and you are thrusting from your pelvis on the way up instead of from your back.
- practice doing unweighted squats just to get the form --
- do box squats -- set a box or bench underneath you when you do the squat -- a "complete" box squat is when you just touch your butt on the bench and then explode back up. Its not meant to sit on - frankly that just messes up my motion. But it gives you a safety point in case you feel like you are going to drop. So work on controlling the downward motion, light touch and then strong back up. When you can do these w/ control and tight form, you can get rid of the bench, lower the bench or whatever. Its a really great tool to get you to the next level of good quality squat.