Jane, your question arrives just as I'm considering mirrors anew.
Let me first of all give you the good news. The mirror I like best is cheap and available everywhere.
http://www.sjscycles.co.uk/cateye-bm-300g-race-mirror-left-or-right-fitting-prod13904/ This is the Cateye 300 G, which has a good field of view without misleading you that cars are far away when in fact they're chomping on your rear tyre. What it lacks in adjustability it makes up in survivability.
It fits either upright or hanging down under your handlebars or sticking straight out, depending on where your hands need to go, and is exceedingly useful. When you park the bike, your just twist the mirror on its ball to align parallel with the bike.
Recently I got new handlebars with the n'lock stem I've described elsewhere on this board and they have a shorter straight section so that with my Rohloff gears I had to shorten the grip on one side, and consequently my hand interfered with the mirror.
To replace it I got this M-Wave 270020-3-D Mirror, bought on an impulse.
http://www.google.com/products/catalog?client=safari&rls=en&q=M+wave+mirror&oe=UTF-8&um=1&ie=UTF-8&tbm=shop&cid=4216499524569876610&sa=X&ei=ttDTT5KcF9CxhAem0LHwAw&ved=0CF8Q8wIwAg#ps-sellers It is, not to put to fine a point on it, lethal. It is so 3-D that it shows cars as half a mile back, at least, when in fact they're already nosing up next to your rear wheel. You really don't want to look in this mirror and think you have plenty of space to turn across the the traffic...
Definitely not recommended.
Meanwhile, until I can find another mirror, or decide the 300-G, upside down, is the best I can do, and fit up my spare (I gave the one I took off to a pedal pal who turned up to help me wrench), I'm using the helmet mirror on my Bell Citi.
http://www.google.com/products/catalog?client=safari&rls=en&q=Bell+citi+mirror&oe=UTF-8&um=1&ie=UTF-8&tbm=shop&cid=6794793850010023785&sa=X&ei=QtjTT-jrBIa5hAeEg-TCAw&ved=0CHgQ8wIwAQ#ps-sellersUnlike several other helmet mirrors I tried, the Bell has enough adjustability to give me a comfortable glance at the road behind me, but unlike Dan I don't like using it much. However, it is amazingly sturdy, the one on my helmet as an emergency measure (in case my handlebar mirror gets knocked off) has outlived two Citi helmets (one crashed and disposed of, one given to someone who admired it). Recommended but unfortunately it is proprietary to the Bell Citi/Metro helmets (also recommended as smart, cool in the physical sense, and very, very ergonomic), though now apparently extended to the Muni and Arella, about which I know nothing.
If I find no other mirror that works for my new handlebar configuration, I might be forced to return to that versatile old standby, the Zefal Dooback.
http://www.zefal.com/fr/retroviseurs/79-dooback.html This is a truly great mirror, and I used it for years. If it is so great, why did I stop? It's expensive to buy and the design, while at first glance sturdy enough (there's no shake in it, once properly mounted) deals with knocks by popping out the mirror, which is glued onto some pins. There's no way of repairing it. I got fed up with going through three or four of those a year. There's a footbridge where I arrive between the stone pillars anchoring it at speed -- losing three mirrors in three weeks was the final straw. But it works, in being infinitely adjustable, and giving you an excellent view of traffic behind and beside you, with no possibility of confusing the lanes. A brilliant mirror for heavy traffic but overkill for my quiet lanes.
the Zefal Dooback is handed, so order by the side you need.
http://www.sjscycles.co.uk/page/find/?name=zefal%20mirror&page=1Notice that none of the mirrors I recommend strap onto the handlebars. A permanently installed mirror should bolt into the end of the bar, or onto the bar.
Finally, for years on my mountain bike I used a truck mirror on a long bent arm, sold by my LBS with bicycle-suitable fittings. I suspect you're a roadie, and thus by definition a weight weenie, but if weight doesn't matter to you, and cost and getting a really safe view of the car behind you does matter, a truck mirror works superbly well. I stopped using those because even the local blacksmith aka bike mechanic, never before suspected of taste, pointed out that they looked cheap on my German/Dutch custom bike.
Maybe I'll try the Zefal Spin.
http://www.sjscycles.co.uk/zefal-zefal-spin-mirror-prod20623/ Andre Jute