I have trekking bars on my commuter and after trying several clamp on mirrors I said forget it. I don't think they make a mirror that was intended for use on butterfly bars. That said, I really like the glasses mirror I have. Picked it up at a garage sale for $2. I wear sunglasses whenever I'm on my bike, and it fits right on no problem. At night I wear a pair of safety glasses and I had trouble getting it to fit those. There's not as much traffic at night and you can pretty much tell where a car is by how bright their headlights are beside you so I'm not too worried on that.

That messenger mirror mentioned above looks pretty good, I think I'm going to try one of those too.