In addition to the Mirrycle I mentioned above, I've also used a home-made invention that involved attaching one of those small wide-angle mirrors to the top-side of my cycling glove. This gave me the flexibility to move the mirror around and capture any part of the road I needed to see. Of course, the downside was that I had to remove one hand from the bar to use it, a bad idea when rolling downhill really fast or cutting a tough corner. Lots of people copied me, though. I wonder if they are all still alive?