I was thinking about what your saying about having correct form. Really you won't know unless there is someone that understands the lift and is watching you. I agree its more of a mental part than anything and any normal person who performs the lift will think they are doing it right.

You said lifting with your shoulders is a main key to this exercise. I sometimes find myself pushing with my heels half way then the other half with the assistance of my upper body.

Now what I try to do is keep the bar on the upper part of my shoulders then decend relaxing my body and letting the weight drop to the desired depth and lift with everything i got. Now when the weight is gets heavier thats when that half feet/upper body process kicks in to get the lift done.

Not sure if thats all correct or what not but I know something im doing is affecting my feet. Although the last couple of days has been better than it used to be.