Re: sorry.. yet another squat problem
A video would really help--side shot would help examine the lean.
Do you force out your abs/gut when you squat? That will help with upward stability.
When you come out from the bottom position do you start with your hips or drive your upper back into the bar? If you are getting significant lean I suggest you start by driving your back into the bar instead of driving with your hips. If you overdrive with the hips, it can pitch you forward into a good morning like position.
Also, how high are your elbows? If you are really torquing them back and up, that can make your chest collapse and make it easier to lean forward.
Ways to help the lean--front squats, safety bar squats, good mornings and squats starting at the bottom (Anderson Squats).