I use two mirrycle end mounts on my flat bar both turned down and in. Driven a car too many years to trust my other senses. I like to scan constantly who is coming up behind me and if I feel they may not give me much extra space. Will still shoulder check even if I'm certain no one is there, but I also stop at stop signs and red lights when other bikes fly throug so maybe I'm overly safe.

I'd get a helmet mirror but I'm sure I'd break it.