I drive a school bus, and I park my bike inside the garage. One day, I returned from my route to find some smart-ass teenager riding it. He was my boss's son, so I didn't make an issue of it. But from then on, I locked it with a very inexpensive $5 cable combination lock from WalMart, which I can attach/unlock in 10 seconds. It's a 3-number combination, but I only move one number one notch when I lock it, so I can get it on and off fast. It could be cut very easily, but in low risk areas, it does the job. When I'm in a high risk area, I use a U-lock, which takes a bit more time to get on and off.