FWIW, I use a Cycleaware Reflex helmet mirror. Many people use the Take-A-Look mirror; I had one for a while and while it's a very effective mirror, it gashed my face up badly enough to require about a dozen stitches when I crashed a few years ago. The Cycleaware mirror has protective rounded plastic around the mirror, the mirror is in a ball socket that easily pops loose in an accident, and the support wire is flexible and covered with rubber.