Hey Fellas i just wanted to throw this in because i thought it was important. I trian by myself for the most part and don't have anybody to look at my form very often. Although i have been doing these for a year or so i still find that i don't always do them with perfect form. I have found from my experience and from watching others that one of the most common mistakes is not arching the back hard enough.
Today in the gym i was trying to coach myself to get back into the best form possible and the only thing that i find is that i have little or no arch in my back. today i worked on it and found that it is so much easier and effective to sit back to where you put all the emphasis on your glutes and hams when you have a hard arch. After all , one of the main reasons to do the box squat is to work these muscles and to sit back.
the reason that i don't all the time is because i don't feel as tight when i do it although i do sit back.
I just wanted to throw this out there for those of you who train by yourself and don't have someone to watch you. Remember to arch hard and sit back and the movement will become much more effective.
Just as a side note , I find that when you have a partner that can tell you to , sit back , push your knees out , arch your back , spread the floor , ECT. that they are invaluable to training properly. Try to tell yurself one or two things right before you lift to focus on and it can help you right away. Today i said to myself "arch your back!!!" before every lift.