Raptor,
Tight hamstrings are the major thing that can push you forward in the squat. As your hips go back your hams have to be "loose" enough to allow that. If they're not you lean forward a bit more to compensate. However, based on some other things you've said in other threads I know for a fact your hip flexors are tighter than piano strings. Particularly up over your belt line. You need to do lots of twisting type hip flexor stretches that strongly stretch your hip flexors up in the outer/lower ab region. Try lying on a basketball sideways. The ball should be just above your belt line and you should be on your side. Reach up and away while simultaneously pushing your hips down in the opposite direction. You will probably hear some crackig and popping in your ab region from the sound of hip flexor adhesions releasing.
I'll try that. As far as my hamstring flexibility, I can touch and even go beyond "floor" level in a standing hamstring stretch. The hip flexors are tight as shit though. Although I can do the regular hip flexor stretch "well enough" there's a lot of pain in my abs when I do it. In fact, if you watch me sit in a chair at my computer I'm very hunched back and the shoulders drift forward because it's just "easier" to sit that way. If I stay straight up I usually get tired quickly or just forget about it and hunch back forward. Probably another sign of tight hip flexors or abs or whatever. Not to mention that probably kills my glutes.
Also I would guess your thoracic area is tight as hell. What happens when you do a front squat? I would guess the bar falls off your shoudlers at the bottom because you don't have the thoracic mobility to keep your shoulders back. Front squats BY THEMSELVES are the best thing to fix that as they both stretch the tight muscles and strengthen the weak ones. Do a couple of light sets a couple of days per week consistently...they don't have to be heavy.
I can't do front squats. First off, I can't hold a barbell on my shoulders, I can't have the elbows forward, I need to keep the barbell in my hands with the elbows oriented down. I just can't do that unless I want the barbell to sit in the middle of my neck. It doesn't matter what grip I take, it won't happen. So no, I can't do front squats. If I try to do them though, keeping the bar in my hands, I QUICKLY (think as soon as 1/4 squat) start to bend forward at the waist. Same happens in the overhead squat. Maybe I'll film once just for... "fun".
But anyway - for you in particular I would do a daily dynamic warmup of broomstick overhead squats, step back lunges with a broomstick held behind your back. The hand that is on top of the broomstick should be opposite of the lead leg. So if your right foot is forward your left hand should be on top. Drop back into a lunge while simultaneously keeping your upper body as erect as possible and slightly twisting. Also do straight leg kicks for your hamstrings and some type of dynamic calf stretch. That'll cover most of your problems.
If you do a lot of sitting throughout the day you have to do plenty of stretching to coutneract that or it'll become problematic.
What do you mean "the hand that is on top of the broomstick"? Ain't I supposed to keep both hands on the broomstick in the same position?