Bike theft is a terrible thing, not least because its quite easy to become attached to the bike which has been almost an extension of your body since a few weeks after that first ride. Its also quite galling because you know that the police, through a combination of lack of resources and apathy see bicycle theft as a, “non-serious crime.” Obviously it helps to have bike insurance, make a note of your frame number (stamped into the bottom bracket shell) and possibly register it with a service such as Immobilise (this should stop some of the less dodgy pawn shops from buying your bike from a thief). The main line of defence is the lock, but even then the best locks can be let down by the stupidity of their owners:
Take this classic example, the D-lock was a good choice, but just slinging it around your top-tube and through a Sheffield stand doesn’t magically protect the rest of your bike components, especially if you have quick-release (“easy-steal”) wheels.
Sometimes I see the D-lock through top-tube lock in conjunction with a cable through the wheels, which is a slight improvement but brings me neatly to my second point; D-locks aren’t invulnerable to leverage attacks, and leaving a big section of your “D” not filled-up with bike just makes it all the easier to break it open with a scaffolding pole and 15 seconds to spare. Its also worth remembering that a thief will choose the bike where the leverage attack on the lock won’t leave the bike un-rideable or in need of repair, so the top tube is the worst place to thread-through your D-lock (apart from the front-wheel only; around Manchester you’ll see the occasional colourful-rimmed hipster wheel alone on a Sheffield stand, its owner having seen fit to secure the rest of the bicycle with nothing more than the front wheel nuts or QR skewer).
The best way I have found to use a D-lock is to put it through the seat stays and back wheel rim and Sheffield stand, and use cables to lock up anything else you’re not willing to lose. This has the effect of reducing the leverage attack area for some scumbag with a scaffolding pole, whilst giving me the satisfaction of knowing that if they do get my bike, they’re probably not riding it anywhere anytime soon.