I have taught people how to squat, dead, and bench, and the trick I have found is to tell them to keep going down, down, down, when they are on the way down. Quite often people think they are lower than they truly are. Anyways, that is how I do it, and it seems to work very well. It also motivates me when people listen and do things like that correctly, so I find I can benefit from them working out properly in my own way too.