I carry one of these: http://www.abus.com/eng/Mobile-Secu...ecurity/Locks/Special-Locks/Combiflex-Pro-202 and thread it through an immobile part of the bike and then through the viewport of my full-face helmet. Thieves happily slice the strap of a helmet, even though that makes it next to useless, at least a 2.5 mm thick wire requires some sort of wire cutter, won't do it with just a Leatherman. It's mostly to keep honest people honest though, but then again so are all locks.