I use a 'Blackburn multi mirror' on my MB. It fits into the bar end, doesn't interfere with hand positions, and is really stable during the ride. Hate riding without it. They also make one for road bikes that also looks like it wouldn't reduce hand positioning options.
http://www.performancebike.com/shop/...tegory_ID=6700

Q: Anyone use helmet mirrors? I've been thinking of giving them a try. I'm leery of the idea of glueing something to my helmet, I'd probably constantly whack it against things.