I think it was Rich who advocated the "Take A Look" style helmet mounted mirror. Instead of circular they are fairly large and rectangular in shape. They give you the 'Big picture,' with a quick glance. Would not leave without it. In fact when I have- I usually return home to get it.
It makes scanning traffic so much quicker and thorough. Just hearing what is behind you is not enough. If you hear something sounding like it is accelerating, better use your full mirror earlier on, in order to ditch your bike- away from traffic. When I turn my head to see what is behind me, it seems the swerving of the bike, at time- is almost enough to loose control.