"If this bike has not been brought into your shop by [insert your name] then it is stolen, please call me immediately on [insert your number]".

These little pieces of paper are in places like the stem, the bars, under the bar tape, in the bottom bracket, under the inner tubes, under the rim tape, in the hubs. Literally anywhere I can put them where they are likely to survive undetected by a casual user of the bike but that is likely to be discovered the first time the bike is taken into a shop for servicing. Obviously I use little bits of plastic and a permanent CD marker for the places likely to get wet (the ones under the innertubes).

I don't expect to get the bike back days after a theft, but weeks or months later I think it would work well.