To make sure the mirror is aimed properly, first point it at the ceiling. This ensures that you're pointing the mirror straight back (at the car), then wiggle the mirror up and down so the reflection of their headlights bounce on/off the ceiling. They will be getting a major flashing in their eyes.

This usually works to dissuade tailgaters who are doing it unintentionally (they just drive up to whatever is in their way, and repeat until at their destination), even if they don't have their high beams on.

On the highway, if someone is traveling really slow, I avoid doing this. Having dealt with elder folks, I realize that many of them simply cannot see that well. The debate on whether or not they should drive is a different topic, but I for one will not try and make them crash. Plus I figure they're at a huge disadvantage due to their frailty - their airbags will probably kill them.

However, a fast driver, or a young one (if I can see them), or a distracted one (someone on cell phone), they get the full benefit of the mirror flash.

cdr