[my cut&pasted reply to a thread on the commuter forum a while ago]

I use a helmet mount mirror, which can also easily be attached to any glasses. I've also mounted it to the brim of a hat on occassion while on tour.

A decent review of various models can be found on the Icebike site. The model I use is the "take a look" model. To mount it to a helmet, there is a small plastic plate that sticks to the helmet-I just attach mine to my helmets visor.

I think that this model is excellent, it hardy vibrates+I rarely need to re-adjust it. I've previously used the plastic hemet mount types, and found they vibrate too much and lose there adjustment easily.

So far I've lost two. One popped off when I went down in deep snow last year (I didn't notice 'till later), one popped off due to the vibrations of rough roads while cycling in Romania. I wasn't wearing my helmet at the time-it was strapped to my rear rack+I didn't notice when the mirror fell off (I had a spare).

Mirror or not, you still need to turn your head and look/check prior to changing lane position.

The thread can be found here

[edit] Adding link to the icebike review page of mirrors (both helmet&handlebar). See: http://icebike.org/Equipment/cyclingmirrors.htm