I use one of these:-
http://www.merlincycles.com/cycle-aware-heads-up-eyewear-mounted-mirror-56442.html
I wear it on the RH arm of my (prescription) glasses for both UK and European use and in use in Europe it makes no difference it being on the 'wrong' side. I don't use it on LH side as it is a little bit 'obvious' as I'm left eye dominant [1]. When mounted on the right hand arm and with both eyes open the mirror is virtually unoticeable, but if I close my left eye then the mirror view 'appears' without actually looking at the mirror (though that works too). Since my head is steerable the mirror can be made to point anywhere I want with only very minor movement, it is surprisingly easy to look anywhere after only a little practice though only rear view is much use on the bike, but I can keep an eye on my parked bike whilst sat in a cafe without turning around and suchlike.
I found bike mounted mirrors could only see what was in view at the time you looked at them - and that was never where I wanted it to be!
You really need to have a bit of your head visible to confirm the mirror is looking behind - if you point it without a reference point then if it gets knocked you can't be sure you are actually looking where you thought you where originally.
Before buying you will need to try one, and try it on both glasses arm to check if you are left or right eyed, and some people can't seem to cope with them on either arm at all, problems appear to be the inability to ignore the mirror (no dominant eye?) or bizarrely claustrophobia occasionally??
BUT always shoulder check!!
[1] make a circle with index finger and thumb and look through it at an object and close your left eye - if object jumps left you are left eye dominant and probably need the mirror on RH arm. If it moves right you are right eye dominant so may favour LH arm. If it doesn't move you have no dominant eye and may be able to go both ways - or neither!
Edited to add SHOULDER CHECK