I use a bar end mirror, which is absolutely fantastic (it was actually on the recommendation of a TE lady!):



I've converted several other riders into getting one - it's discreet and has a very wide angle of view.
It's great for group riding, as you can intermittently check what's going on behind - and if you're at the front, you can check that your pace is ok.
Initially club riders laugh at the geekiness factor, but then start asking where I got it from!

I should point out, though, that the mirror doesn't just pop into the plug at the end of the drops, you need to partially unwrap the bar tape (but only partially).
It's also advisable to get someone to help you fit it (to tape up the mirror while you check the angle).

The mirror is available from a company called Aspire Velotech:
http://www.aspirevelotech.com/Mercha...ry_Code=MIRROR

The service is excellent - delivery was very speedy, even when I was living in New Zealand.