Make sure you are keeping your back arched when squatting. Don't think of it as squatting, like squatting down...I don't know...how sedentary individuals would describe a squat, like not a lifting squat. Think of squatting as if you have a chair behind you and you are sitting down into it. This helps me keep my knees behind my toes. Also, keep you eyes forward, not down. This should keep your back arched better. Lastly, I agree with fhg43 to a point. Don't do squats and deads on the same day. I have a pretty strong back, but still don't think it would be a good idea to put my body through that. Also, you can't possibly go all out through sets of both in the same day!
NateDogg