Rear view mirrors are much more effective when attached to the down tube, right where it meets the head tube - this gets rid of some bar clutter, and also means the mirror is more fixed - i.e. it is always looking behind you, and doesnt move with turning the bars. The down tube is at just the correct angle to get a really good view of the road behind you when viewed from the riding position.
I initially tried it as I moved to trekking bars and had no bar ends to fix my mirror to - and I couldnt believe how much better sighted I was for the road behind. Fixed in place with a couple of cable ties.