A full-length mirror can work for you or against you - it all depends on how you use it. It can be a tool of the ego if you spend an inordinate amount of time admiring yourself. It can destroy your self-esteem if you use it as a tool of criticism, picking out every physical flaw as an excuse for self-hatred. Or a full-length mirror can be a tool for optimum health and fitness. How? By using it periodically to check your form when you're working out. When you're performing a squat, are your knees jutting dangerously over your toes? Is that biceps curl really working your bis, or are you using momentum to raise a too-heavy weight? Think about your Yoga poses - how vertical is your spine during Warrior II? How is your posture in Mountain Pose? Is one shoulder higher than the other? Look at yourself from the side - is your body properly aligned?