I've recently noticed that my squat form is less than perfect. My back tends to lose its natural S-curve at the bottom of the lift. It's not very noticeable (I didn't notice it until I was body-weight squatting without a shirt on next to a mirror), but I would like to fix it nonetheless.
As I approach parallel my lower back starts to round ever-so-slightly (my back's still straigth, but it does not have the natural curve in it) and gets worse the further below parallel I go.
Once I noticed the problem I tried to fix it immediately. I threw a hockey stick over my shoulders and started squatting. Whenever I kept my back perfectly straight I would lose my balance, falling backwards. Obviously I don't want to do that with any weight on my back.
I squat with a slightly wider than shoulder width stance.
Any tips? I can post a video later if necessary.