It seems like no matter what you do bicycles are just too easy to steal. As long as a thief has a bolt cutter or a crowbar, they can get through most locks in a jiffy. Cars have alarms to deter thieves, but why not bikes? Here's an easy, cheap way to create a magnetic bike alarm that will leave thieves fleeing the scene.
You'll probably want a bit of electronics experience to put this together, but it's hardly a complicated project. Essentially what you're doing is creating a tilt sensor, that sets off a loud noise when it shifts from one angle to another. If you aren't super-experienced with electronics, you could probably buy a cheap tilt sensor as well, which might take some of the legwork out of the process. Check out the video above to see more, or hit the link for the full how-to.