I use an old ski lock that I had lying around. It's basically a thin cable combination lock that "zips" out kind of like those lanyard badge holder thingys. I put the helmet on the handlebars, loop the lock through the chin, and around the handlebar between the clamps. My cable lock is about 4', which can even allow me to run it through my jacket arm, helmet, and handlebar so I don't have to carry all that crap wherever I go. It would probably take about 5 minutes with a pair of safety scissors to get thru it, but I've never had any problems with doing that.
My goal is simple. It is a complete understanding of the universe, why it is as it is and why it exists at all.
- Stephen Hawking