I highly recommend reading 'Starting Strength' by Rippetoe for pages of great info on squat form. It helped me so much and once I put into practice all that Rippetoe preaches I realized I needed to really improve my flexibility and squat form and to do that I needed to drop back in weight and improve my depth.
In the meantime, I found this link to be informative:
Should I be squatting below parallel? (by Tom Venuto)