I'd like to emphasize that you need to practice on a distant target such that you can't see the reflection hitting the background and guide it onto the target. Otherwise you are not really using the aiming system.

As mentioned before, a retro-reflective target such as special tape, automobile tail lights, bike reflectors or road signs work well for this.

My mirror is a Vietnam-era glass job w/o retrorelective mesh. From as stationary platform I can hit a stationary target very easily. A moving target would be more difficuly. From a moving platform, it would be very tough.