I've ridden bikes with drop bars for about 14 years, and I have never felt comfortable riding without a rear view mirror. Those available in bike shops don't seem to cater very well for the drop bars (perhaps it is not "cool" to use a mirror on a road bike!). They either vibrate like crazy, or fit into bar ends - which means a large eye movement down to look at what is happening behind. To overcome this, I made my own many years ago, and it has been quite successful. Having now bought a new toy with Ultegra gear, I had to re-design the original concept.
Essentially the mirror bit is just a Mirrcycle mirror with the handlebar plug cut off. The important bit is the 1mm aluminium attaching it to the brake hood. It is rolled to fit over the brake lever and under the rubber boot and is firmly held in place by velcro strapping. Up and down movement on the arm is minimised by the depth of the arm, and I have reduced sideways movement by putting a slight roll in the flat metal along the arm.
If anyone is interested in more detail, I'll post it later, in the meantime here is what it looks like.