I have a bike mirror on all my bikes. I've bought a helmet mirror but have yet to really like it. Bike mirrors tend to last a while--a season to several years--and then the bike falls over or the mirror gets whacked while entering a doorway, something like that, and the mirror breaks.
The issue is different with a folding bike, though, because the very act of folding the bike and packing it around or storing it beside the desk at the office offers more possibilities to bump the mirror.
One solution I'm trying now (and it seems to work) was found, oddly enough, not at my local bike shop or on some online source but at the local Wal-Mart. Their little nook of generic bike gadgets had a Velcro, strap-on bike mirror, made by Bell.
I actually can't find online the exact equivalent of this mirror; perhaps is a new version put out by Bell. This mirror has a flex handle, which helps with bumping, and the Velcro securing strap is smooth, lacking a buckle for folding over that I saw from an online photo of an earlier model. Other versions of this mirror have rigid attachment shanks (the more easy to snap) and buckled Velcro attachments. Some users, though, in reviews have mentioned that the rigid shanks are better because the flex version of the mirror vibrates or has to be regularly adjusted. I imagine the bumpier the road, the more this might be true.
So far, my procedures for portaging the bike now include unfastening the mirror and sticking it into my pocket when folding the bike. At $7.96, I'm not sure how durable the mirror will be, but even buying one a season is certainly much cheaper and wiser than having a headbutt (or tailbutt) with a recreation vehicle.
Check out the photos I've taken of the mirror I use. I think the articulated neck and the smooth grip are key details that led me to try this particular version of the strap-on mirror.
Copyright 2013 by Thomas L. Kepler, all rights reserved
The products mentioned were bought by the author, who is--at least at the time of this writing--not a representative of any company mentioned, nor does he wear the blue vest and work at Wal-Mart.