You aught to get a helmet mounted mirror to be a proper Fred.
I use this:
http://www.amazon.com/Mirrycle-MTB-M...4608366&sr=1-1 mounts easily, steady as a rock and shows what I need to see. I DO mount it BELOW the bar though so that it's not in the way should I have to flip the bike over to change a tube or straighten a chain. It is a good precaution to use a mirror but you should still take a look back over your shoulder when doing something that may put you into traffic.