I always use a bar-end (I think I have the 3rd Eye brand) mirror. I did break one last season and before I replaced it I went out for a ride and was VERY uncomfortable without it, it had become that much second nature to keep glancing at it.
I could NEVER get used to either a helmet or sunglass mount mirror. How do you guys do it? With the bar-end, I just glance down at a fixed point whenever I want and it always shows me the field I am accustomed to. With the helmet or sunglass mirrors, you have to perfectly position your head, line of sight,shoulder position, etc. Way too much concentration required, in my experience.