Long term, I think it's infrastructure changes and a more engrained bike culture that allows people to bring their bike inside with them more often. (Of course, maybe we just bring the thieves indoors then, but I bet the reduction in ease and opportunity corresponds with a reduction in theft.)

I ride two bikes, one worth stealing that simply never gets locked up outside, and another that nobody would ever want nor would I miss. When I ride the former, I only ride to places that allow me to take my bike inside: Roomy coffee shops or offices with storage for bikes.