Since you have flat bars, I will not hesitate to recommend the Mountain Mirrycle mirror: http://www.mirrycle.com/mountainmirrycle.htm

I started out with a helmet mounted mirror, and it did OK, but when I broke it in a crash, I decided to try out a handlebar mounted model. I read a bunch of reviews before deciding on the Mirrycle, which I've been using for about a year now.

It's fantastic. For one thing, it's huge. With a helmet-mounted mirror, you're always tilting your head this way and that trying to figure out what's behind you. Not so with the Mountain Mirrycle. It gives you a big, easy to see view. It also doesn't vibrate. The arm is bulky and more than a little ugly, but it gives you a nice range of adjustments and once you've put it in place it stays there.

Now, after gushing like that, a word to road bike riders who read this: I've gone with a bar-end mirror on my road bike, and I can't say it gives me anything more than a hint as to what's behind me. Cars with headlights on I can spot pretty well, but dark-colored cars with no headlights can hide in shadows pretty well.