A quick-n-dirty summary of essential squat technique:

• Absolutely critical that you go below parallel, where below parallel means the crease in the front of the hip goes below the top of the kneecap.*
• The femur should track parallel over the foot - the thigh bone and feet are pointed in exactly the same direction, each about 30 degrees outside of straight-ahead. The knee should track over the toes.
• Heels on the ground! Weight on the heels! This is critical.
• Your back can bend forward about 45 degrees. Some people think it needs to be straight up and down… no way!
• Maintain lumbar arch, do not round your back. Bending forward only means hinging at the hips; your back should remain straight.

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* If you stop the squat above parallel, you’re using the quads but not the hamstrings-and-glutes. Developmentally, this is going to pull your tibia forward on a regular basis and develop a musculature that pulls the tibia forward, which means shear force against the ACL, the anterior cruciate ligament. The hamstring balances this with backward pull against the tibia and thus balances forces on the ACL. Deep squats, below parallel, are therapy and prevention for this common knee injury.