I used to have a glasses mirror, but then when I saw one of our elite riders in my club with a mirror on the drops, I decided to try that.

I now have these mirrors on both of my bikes. What I like is I can see behind me for probably 1/4 mile or more, so I know when traffic is coming. It's specially nice when they have their lights on!

When I am going to move into another lane to turn left, for example, I always look over my shoulder (as I do when I drive as well). It's easy to adjust which I will often do on the road.