Concentrate on what you're doing. Before doing a set, take about 20 seconds to think of what you're about to do. If you were talking between sets, you must stop talking, and focus on the task at hand for at least 20 seconds before lifting, or your set is doomed. I took a graduate course in sports psychology under one of the best sports psychologists in the business-Bob Rotella, at the University of Virginia. One of the most helpful things I learned was that there are two main types of concentration.
External concentration is the type that's used by actors and performers on a stage.
Usually outside cues are given by a director or conductor, etc. Peripheral signals, timing, and non-verbal communication by other actors, for example, must be noticed by a broad view or an outward concentration. This is exactly the sort of concentration you use when gazing into a mirror. If you feel you absolutely must use mirrors to learn form, use a bar with light weight to practice form, but this should not be considered actual training. Most mirrors in gyms are not full length anyways, so you don't even see your lower body.