Most individuals think that mirrors are there to evaluate their technique. Although this may sound effective as it gives immediate feedback, it does so in the wrong way. Kinaesthetic sense (body awareness) is an attribute which separates an amateur from a professional. When you perform a movement, you want to be able to correct it based on HOW it feels, not through visual feedback. In addition, when your focus is shifted on an external object (ie - the mirror), your concentration deteriorates, which is the last thing you want when lifting something heavy.
Still not convinced? The mirror only gives you feedback on the frontal plane. It has no analysis of what the movement looks like from the side, and that is sometimes what is most crucial (ie - back posture during a deadlift, or depth during a squat). For those that are smiling, and thinking 'Thats ok, my mirror is on my side so I can see what my squats look like', then also think about turning your head while lifting anything is extremely dangerous.
Listen to your coach or trainer. Their corrections are helping you learn how to control your body better. If you don't have a trainer with you, ask the floor staff at the gym. Don't be shy, it's their job! If you are by yourself, analysis via video is also effective. Tape your set, analyze it and fix your mistakes.
You want to perfect your movement yourself, not by the mirror. You want to feel the error, not see it. After all, which sporting competition has a mirror in it?
