Bike theft is very aggravating when personally affected. It is also a factor that damages the sustainable bike transport sector as a whole. Fear of theft can hinder people of buying or using a bike.
While the danger of theft can never be completely eliminated, it can be vastly reduced.
You should always lock your bike to a fixed place when not using it (bicycle stand, street lamp, tree).
Putting a code on your bicycle frame can discourage theft and help in the recovery of a stolen bike. Most often the name and address of the bike owner is engraved in an encoded form into the bike frame. But it is a relatively complex procedure and some have concerns about privacy or damages to their bikes (carbon bikes, very thin frames).
What every bike owner already can easily do and should do is to make a picture of his bike and note down frame number and other characteristics of his bike (maker, wheel size, color). This data can help a lot when trying to recover a stolen bike. Today, most do not know the frame number of their bike and cannot specify it in a theft report. During police controls or when buying a used bike checking the frame number will not yield any results even if the bike was indeed stolen.
There are several measures to make frame numbers more efficient for theft prevention. Firstly the producers of bicycles should only use exactly one number for each individual bike and put it easily visible onto the frame. Currently the frame numbers are not always distinct and are often hidden below the bottom bracket. The police should publish the frame number (and pictures) of all bikes reported as stolen. A quick online (Google) search for the frame number will thus be able to confirm if a specific bike was stolen or not.
A sticker on which the frame number or code is written with a waterproof pen can discourage theft. Even if the writing or the whole sticker might be removed with a bit of effort the potential thief will know that the owner of the bike has at least noted down the frame number of the bike.
There are „single trip thefts“ where the bike is thrown into the next bush after riding it once. The sticker will make it easier for the finder of such a bike to check the frame number.
The sticker (3.5cm x 8cm) can be downloaded as a high resolution png or svg file and be printed on a weather-prove adhesive foil. As this mostly only pays off for high quantities the sticker will probably be made available through this website, too.