Try front squats. They'll strengthen the abs, and will reenforce proper posture during squatting in general. On a front squat, it's impossible for you to go too far forward really, as if you go too far forward, you'll just drop the bar. Also, if the weight gets unsafe you can ditch a front squat easier.
Also, you should probably be deadlifting to strengthen the lower back musculature. You can also do hyperextensions and good mornings, too, but deadlifts and squats sorta feed eachother.
So do front squats for a couple weeks, then go back to back squats, and you'll find you'll be able to back squat a lot better. Also, front squats directly improve your ability to clean, too.