I found this video to be especially useful. It's Dan John teaching basic squat form, and teaching you how to teach squat form. The secret? Goblet squats and potato sack squats.
He also gets into the starting position for the snatch, one of the two competitive Olympic lifts. It's a great exercise to learn.
The video is 50 minutes long, but it's really excellent. If you squat, it's worth taking the time to watch it. One warning - It cuts off at the end - the person filming it ran out of memory or battery, I don't know which.
Dan John on the Video Fitcast
Why should I learn to squat? I'll defer to my friend Jungledoc on this - he's got a great post about squatting - how and why.
Who is this Dan John guy? Interestingly, Boris Bachmann just wrote about Dan John on his SquatRx blog. I'll let Boris explain why Dan John is worth listening to. The short version - Dan makes the complicated simple.