Don't use mirrors too much. In fact, you should do most of your exercises facing away from any mirrors.
A mirror only shows the front of you. If you stop looking into a mirror, you'll find that your ability to "feel" your form through proprioception becomes significantly stronger. And it will tell you things that you simply cannot view in a mirror. Try doing an entire session without using mirrors; if you find it difficult or impossible to maintain proper form, that should be an extremely compelling sign that the mirror has become a crutch, and not a very good one.
There's also the unfortunate fact that, by looking into a mirror, you're guaranteeing that your head is tilted farther back than it ought to be for exercises like squats. You never want to have your neck extended while carrying a significant load. It should be in a neutral position — for a squat position, this will equate to looking at the floor roughly five or six feet in front of you.
A mirror only shows the front of you. If you stop looking into a mirror, you'll find that your ability to "feel" your form through proprioception becomes significantly stronger. And it will tell you things that you simply cannot view in a mirror. Try doing an entire session without using mirrors; if you find it difficult or impossible to maintain proper form, that should be an extremely compelling sign that the mirror has become a crutch, and not a very good one.
There's also the unfortunate fact that, by looking into a mirror, you're guaranteeing that your head is tilted farther back than it ought to be for exercises like squats. You never want to have your neck extended while carrying a significant load. It should be in a neutral position — for a squat position, this will equate to looking at the floor roughly five or six feet in front of you.