I used a Bell helmet mirror for a short time but it was never in the right place...being attached to my head. I then used a Zefal bar end mirror which was alright but was obscured by my forearm when using the drops. I then tried the Blackburn mirror that mounts on the brake hood and it is the best so far. One added advantage is that it can be moved from one bike to another in about five seconds and it also works as well on the Bull Horn bars on my single speed as it does on the conventional drop bars on my other bikes. Here's a link to the mirror. Naturally, now that I have already bought one at full price, it is now on sale.
Blackburn Mirror
Dogbait