Helmet mirror rocks. It doesn't bounce around, and gives you a view of the entire road behind you. If you have it setup just right, you almost don't even need to turn your head, just glance up and to the left and there it all is.

It might make your eyes feel weird for a week (but only when you're on your bike), but once you get used to it, it becomes second nature and you'll start trying to look into the mirror even when you're off bike walking around.