I was actually just reading 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 was actually just reading this yesterday, and I think it contains the answer to your question (and the answers to several dozen hundred other questions).
I've been trying to improve squat technique for a while. I've found "Squat Rx" on youtube ( Squat Rx #1 - YouTube ) helpful.
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.
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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.