"Bike theft isn't just a nuisance. It's a serious problem plaguing almost every city in the world, and it stifles bike-friendliness, discourages people from riding, fuels crime and undermines our sense of community. Yet almost no cities are doing anything to seriously tackle the problem. "

This is a great video from Shifter about the benefits of bicycle registries. If you cycle in an area with a bike theft problem, chances are the police recover a lot more bikes than they can ever return because they simply have no way of knowing who to give the bike back to. One tool you can use to protect your bike is to register your bike with a service that the police are able to access. Tom's video focuses on Project 529 Garage in Canada, but there are similar services worldwide. There's a list here on wikipedia.

Good luck out there!

https://en.wikipedia.org/wiki/List_of_bicycle_registers