Defending Against Surveillance?

Extrudedaluminiu asks: "With the recent news about domestic spying by the NSA, American citizens are put in a very difficult situation. Citizens in other countries, around the globe, also find themselves in situations where their lives can be examined by government agencies or other groups of questionable ethics. What can people in this kind of world do to defend themselves? Are there any approaches to thwarting or mitigating surveillance that will work on a mass scale? What technologies can people use to hold on to their freedoms, in a difficult world?"