When I carry a lock (such as when I got to the doctor's office) I carry both a cable lock and a small bulldog u-lock. The bulldog goes around part of my frame and my rear wheel. I then run the cable lock through the rear wheel, frame and front wheel. Both locks are also threaded through something physically unmoveable such as a bench or bike rack.

I do this because my understanding is typically a bike thief has the ability to open one type of lock but very rarely able to open two types. I have yet to have the bike stolen <knocks on wood>. When I commute to work I don't carry a lock at all. I've rolled the bike into McDonalds, 7-11s, etc., with permission first though.

Mel