Best thing would be to post a video so people can really see how significant, if any, the problem is. Next, watch some of the videos that are in the sticky above, especially those that Glenn Pendlay has commented on, so you can compare and see whether you have a real or imagined problem.
It could be a lot of things. One big one is simply a matter of flexibility, IMO. How long have you been practicing the squat? Are you doing an oly high-bar style squat or more of a powerlifter, wide stance squat? Are you falling forward? Depending on your squat style, you may want to lean forward some or you may want to stay pretty vertical.
Could also be general hip/hamstring flexibility, which you can improve just by doing the movement and focusing on keeping the arch tight using moderate weight.
Losing it on the descent . . . do you lose it right at the very bottom (not uncommon), or midway? Where are you elbows during the squat? And hand spacing on teh bar? May seem silly but in my experience, widening my hand spacing on teh bar helped my balance and helped me keep more vertical. Also, when elbows start pointing back rather than down, you tend to fall forward.
Post a video if possible.