I've always worked on the theory that someone can steal my bike provided I never find out who they are !
Unfortunately this is one of those things that you really cant do much about, if some scumbag is intent on nicking your bike there is not much you can do.
I'm always pretty carefull where I leave my bike, for instance if I stop at a cafe for a coffee I park my bike right in front of the window where I can see it. I never park a bike in a street and walk away and leave it. I guess I am lucky living in the country rather than a city, less chance of a bike disappearing.