When I teach travel safety classes, I always get lots of questions about kidnappings. One must be alert to the possibility, but there are lots of tactics that you can employ to reduce your risk. I have an entire chapter on the topic in Choose Adventure: Safe Travel in Dangerous Places.
In addition to the mitigation tactics I describe in my book, I also find it helpful to understand how kidnappers get paid and their general business structure. The article linked below provides some fascinating insight.