This is so important. It's incredible the difference it makes to the difficulty of the exercise let alone the damage you prevent to your body.

What would be awesome to take this idea a step further and create a Kinect app (or perhaps some other related/similar tech) which could determine if you are executing the correct form and make suggestions about how to correct yourself. This could also be used to monitor your progress in getting more flexible (e.g. in yoga).

EDIT: Obviously, not applicable to this app, but the idea stands nevertheless