It seems a lot of form problems with the squats are caused by inadequate flexibility - I think mainly in the hamstrings and also the shoulders.
What are some good stretches that can be used to enhance the squat form?
Mobility in the hips is often just as much a problem as tight shoulders and hamstrings. Squatting early and often helped me.
I improved my squat by:
More recently, my squat form has been tremendously improved through the use of a variety of mobility exercises and stretches, such as:
For shoulders, shoulder dislocations: http://www.youtube.com/watch?v=33P5AI27eiU
For hamstrings, here are two:
Tight hamstrings can make it difficult to keep the lower back in extension, and make it difficult to get to proper depth. Shoulder flexibility issues will prevent you from using an optimal grip on the bar.
You are right about lack of flexibility causing problems with your form with squats. In addition to your hamstrings, shoulders and chest, also consider the flexibility of your:
If you find restrictions in these muscles you can stretch them specifically. Core strength is also important to good form.