I tried "attach to glasses" mirrors and found them to be useless until a friend suggested I try "Bike Peddler Take A Look Mirror" (available at rei and some bike shops). Its design permits easy adjustment.

It still took some getting used to and it's not like having a car rear view and side mirrors but it's definitely useful. It can also be attached to a helmet visor but I've never tried that. (I always wear glasses even if just clear lenses in order to keep debris out of my eyes.)