Originally Posted by Pobble.808

I've had good luck with the CycleAware Reflex mirror - Amazon page is here
It has a plastic ball and socket type mount which makes it easy to detach the stalk and mirror from the helmet. The adhesive that holds the ball piece to the helmet will eventually fail, at which point you whip out a tube of your favorite variety of super glue, which should keep it stuck on for good. This is a helmet-only proposition, won't go on glasses.

I've also used the Take a Look on my helmet visor and it works fine too, but I found that I have to zip tie it to keep it from falling off.