I use a Third Eye mirror on each of my helmets. What I like is the adjustability of the mirror-- back and forth and side to side. It has a pivoting arm post that I can also raise and lower. When I move from the hoods to the drops it's easy to make quick minor adjustments. Of course if I am in a deep tuck I must raise my head to utilize the mirror.

I just stuck the adhesive backing with the base onto the outside of my helmet, then attached the arm with mirror to the post and reattached my visor. I find I need a little more distance of the mirror from my eye to view the best. Also, it seems to stick the best on that surface.

http://www.amazon.com/Third-Eye-Helm.../dp/B0015ZZBM6