More squat volume at weights light enough to maintain correct posture.
The problem isn't flexibility if you can sit into the correct bottom position - the problem is not being strong in the correct posture as Matt said. The only way to get strong in the correct posture is to do strength work in the correct posture. So to that end, you can throw in 3-5 sets of 5 squats each squat workout before and/or after your main squat workout with about 50% of max or so - enough to feel it but light enough to ensure perfect position. And with all squats, focus on moving the knees and hips simultaneously, both down and up.