I really like the "Take-a-Look" sunglasses mirror. It's super light-weight and very adjustable. Rarely shifts at all on the bike (only when you take off your glasses and put them down). The only downside is if you stop somewhere and forget you have it on. I've gotten some pretty funny looks at the farmer's market in my sunglasses (helmet now off, hanging on my bike handlebars), with my rear-view mirror!

Emily