Oh and one more thing about the butt wink, in case that turns out to be the problem. To correct mine, I had to think in my head that my first first movement out of the lockout at the top is with my hips, not a bending of my knees. When I thought of it that way, it was way easier for me to establish my knees out position, and to more easily get them where they needed to be relative to my toes. That made my back angle correct through the entire squat. A big problem I had was actually what IW warned you against which is leaning too far forward, in that I was actually establishing TOO vertical a back angle (I'm talking low-bar position here) and it was putting too much stress on my groin (upper upper adductor to be more precise) because I was essentially squatting straight down. If you look at the angles illustrated by Rippetoe, if you squat LITERALLY straight down from the starting low bar position, there is no way your back angle can be correct. In order to keep the bar over the middle of the foot the entire time, the hips have to pivot first, or you WILL lean forward to the point where the bar itself is too far forward.

Doing this by yourself is very hard, because you can't see yourself squat, and it's even harder if you work out with someone who doesn't know what they're doing.