A Squat is a compound movement; it requires every major muscle group to work in concert and instantly pinpoints if an area is tight and needs to be stretched or is weak and needs to be strengthened.  Test yourself using this guide, then incorporate any needed fixes into your workouts.  Correcting imbalances you can't see will help you gain better results from training and preventing future injuries.  

Back Is Flat

If yours is not:  If your back rounds or arches, blame tight lats and a weak core. 

The fix:  Loosen upper back and lats on a foam roller, and target deep abs and obliques by doing twisting planks: Hold a plank, then twist torso to the right and pause for one count; repeat to the left.  These are also called Plank Hip Dips

Torso Is Upright

If yours is not:  Your calves or hip flexors are likely tight, and your glutes weak.  

The fix:  Stretch and foam roll the calves and hips, and do hip bridges to activate the glutes; Lie face up on the floor, knees bent and feet planted.  Raise hips as high as you can, hold a few seconds, lower and repeat. 

Arms Should Be Straight

If yours is not:  Your pecs and lats are tight, and your traps are weak.

The fix:  Loosen the lats and chest with foam rolling, and do chest-openers, such as standing with forearms against the edges of a door frame and leaning forward.  Strengthen the lats with superman pose; Lie face down with arms extended above head; raise arms, head and chest then hold and repeat. 

Thighs Are At Least Parallel To They Ground

If yours are not:  The deeper range of motion is probably new to you, and you need to build muscle memory to achieve it.  

The fix:  Squat low with a physio ball at hip level between your back and wall. Hold at the bottom for five seconds, press back and repeat.

Knees Point Out

If yours do not:  The muscles on the outside of your thighs are weak; on the inside they are tight.  

The fix:  Target inner thighs with a foam roller, and hold a side lunge to stretch them.  Strengthen side muscles with lateral banded walks; Place a resistance band around and above knees.  Take big side steps to the left, then the right.  

Heels Are Flat

If yours are not:  It could be that your calves are tight, your form is bad, or both. 

The fix:  Foam roll and stretch calves.  (Rolling, as you can see is the key.)  And practice squatting into heels, not the mid foot or toes.  Focus on driving hips back.