When to use a mirror?

It's worth getting to know your body and movements from outside. Look for a whole-body mirror, or go to the gym if you don't have one at home. Take a look in the mirror, remember your appearance and movements. You can do this without clothes at home as well.

You might not be in record-breaking form, but accept your present state without judgement, and set yourself targets, by visualising a person in the mirror who you want to become.

You could correct details of your movements during exercising with a mirror, but I recommend you use it only in the first stage of learning about your movements. When you look into the mirror, your concentration can be distracted. If you don't focus on the exercise you are doing but only your appearance, your effectiveness will decrease, perhaps to less than a third of what you might achieve otherwise. On top of that, if it arouses vanity or discontentment with yourself because of your appearance it will demotivate you further. It's even a bigger problem if you look at your training partner in the mirror - because you may make unfavourable comparisons and lose concentration.