I agree with Peegreen, and am myself a police officer. The best thing to remember about theft is out of sight out of mind. Don't leave your bike out, and don't show anyone your bike that you don't know or trust. I always keep my bikes locked up in my garage and covered. If you don't have a garage, find a friend that does or rent a storage unit. Nothing is full proof, and if a thief wants it bad enough the thief will have it. I keep full records of all my things in the event my property/bike is stolen. A little suggestion for everyone is to write their serial number, model number, year and any after market parts or anything that will help identify your bike down. If you have never taken a picture of your bike, take one and in the event you bike is stolen provide the picture to the officer. Face the facts that some officers will not have a clue on what your bike will or should look like. I'm sure Peegreen will agree, nothing is more frustrating for an officer to handle a theft complaint when the victim does not have a serial number or even a model number on the item that was stolen. Then the victim expects you to pull out a miracle and not only find the item but prove it was owned by them. Before you buy a bike, write down the serial number and have your local law enforcement agency conduct a VIN check to make sure you are not buying a hot bike.
On a better note, use common sense, follow some of the rules above and keep insurance on your bike.