Well you can always do box squats. They are not as effective as regular full squats, but it will keep you straight and your form will be good. I would just keep practicing full squats with light weight until you get the form down though.... it might be a flexability issue... are you very flexible? Can you touch your toes with your legs straight? I have never been able to do and it's led to some problems for me over the years because my muscles are so tight.
Front squats are another substitute as well... and I would say they might be better than box squats. The fact that the weight is in front of you as opposed to behind you makes it easier to balance, and you form should be pretty good. You have to get em in though man... I can't stress it enough.