I used to use a helmet mounted mirror (sticky tape), but it kept falling off. I tried different types of sticky tape but nothing would work for long. One day it fell off an I ran over it with my back tire.
I found one in my LBS that secures either to my glasses or the visor of my helmet.
http://www.amazon.com/gp/product/B00...293TXDFEBN7AT2
Never had any problems with it.