Most of the mirrors I notice around here are mounted on the handlebars.

I happy with my Third EyeŽ Hardshell Helmet Mirror mounted on the left side of the visor. It's steady and provides a good field of vision. The only problem is that sometimes while I'm walking I'll glance up and to the left to keep aware of what's going on behind me. Well, the other problem for some people is dislike of mirrors and of visors.