When you are squatting pull your shoulder blades together back as hard as you can.
A more advanced weight lifter told me to do this a couple weeks ago. I tried it yesterday, the bar felt more stable and now I am sore as hell in my upper back which has never happened before.
Your set up should have your shoulder blades pulled together while you drive your head back towards the bar. Sqeeze the bar as hard as you can. This will keep your arms and back tight. You should have your elbows directly under the bar, not behind or in front of it.
When you start the squat you should push the hips backwards instead of downwards, causing you to bend at the hips and stick your butt out. This will keep the knees from going forward over the toes. While you are concentric you should concentrate on driving the knees outward and pushing your hips forward.