this yesterday, and I think it contains the answer to your question (and the answers to several dozen hundred other questions).
I'm trying super hard to keep good form doing squats. I seem to do best when I use an exercise ball behind me against a wall up near my shoulders (any lower and I can't stick my butt out). But I still can't seem to get good hamstring and glute work. I feel it mostly in my quads even though I try to keep my weight in my heels and push from my heels. Any tips?
Don't let anybody tell you, "You can't" just because they can't.
I think Mark tells you to do it against a pole at the park or something. Check his Squat Progression video out.
Just for the record - I have never found squatting with your back against a ball (or anything in fact) to be helpful in learning proper technique. If anything this causes the torso to be too upright and doesn't allow proper glute activation.
Go to youtube and look up a video series called 'So you think you can squat.'
I used a low box to do box squats to fix my form. Most people have issues "sitting back" in a squat.
A big tip I have to share, and this will save you a lot of trouble and keep you safe:
I can't stress this enough:
Keep your abs as tight as you can when you squat so that you will protect your back, I really can't stress this enough.