Whenever I know I'm going out for a loaf of milk, I use my grocery getter bike rather than a snazzy bike. Oh heck, all of my bikes are cheap pieces of crap, who am I kidding! I really believe that a u-lock will do most of the work most of the time. Or a u-lock and a cable if I'm afraid of the bad guys. I set the bike up so that it hasn't any quick releases on it, and so that I don't leave anything on the bike that isn't secured: I use zip ties to keep the lights from disappearing, for instance.

You want to deter theft? Ride an unattractive bike, and have a lock that's better than the next guy's on the rack. You can't keep a pro from getting your bike if he wants to rip it off. Really, it's the opportunist you deter with a lock -- practically any lock. Realistically, you're only buying time with any lock.