Good advice from everyone...you have to HAMMER your core from all sides. Lower back, abs, obliques...not PRETTY, not CUT, but STRONG. Partial valsalvas too.
All these thing will help you keep your form better, assuming you know what proper form is. Do you ever box squat? CoolcolJ had it right....head drives back, look up, chest drive up to the ceiling. You release the hip flexors and FLEX the hips forward to lift. When you're at the bottom, instead of thinking of the squat as a PRESS think of it as a flex. You are doing nothing more than aiming you chest to the sky, and FLEXING your hips forward HARD to raise the bar.