This article from Precision Nutrition is very extensive and includes trouble-shooting guide, various squatting variations and a lot of other useful informations. Especially helpful if you are a coach.
Highlights from the article:
The squat is a basic human movement. Performing it makes you better at athletics, fitness, and life in general.
If you want to get better at the squat, practice. Practice helps to coordinate movement, and builds the mobility you need to do the movement properly.
Start with a squat progression, such as squatting down to a step. Make it your goal to get as full a range of motion as possible — even though this may take time.
Every body type is different. Try a variety of squats, stances, and ranges of motion.
Focus on form and proper technique, not piling on weight to impress your gym buddies. Check your ego at the door.
Do your mobility drills. A body with poor mobility is a body that will likely get injured with squats.
Full squats are often safer than shallow squats. The deeper you go when squatting, the more muscles recruited.
Control the descent and reverse the movement carefully. Don’t rely on your ligaments to bounce you out of a deep squat.
Think about how the squat helps your fitness and performance — don’t focus too much on how much you can lift. The squat technique that allows you to lift the most weight isn’t necessarily the best or most appropriate option.
Keep it simple. Even babies can squat. Don’t over-think it.
Be sure to check out the full article from Precision Nutrition: