weight under the heels work great, besides inflexibility, bodystructure has a lot to do with your squat form. You also might want to try really widening your stance, this will hit your glutes more. As a stretch... sit in the bottom position and SLOWLY move your hips around a little to loosen them up.

I've been known to do squats in my 5" heels - for practical purposes only!! Really! I had to practice actually being able to function in them - not to mention be able to dance, but it really hit the quads when your heels are up that high, lol.