The definitive literary treatment of a police state is George Orwell's novel Nineteen Eighty-Four, which describes a totalitarian regime that uses the excuse of constant war to permit police and security cameras to surveil the entire population.
A classic modern police state was East Germany, or the German Democratic Republic (GDR). The East German secret police force, the Stasi (or Ministerium für Staatssicherheit) maintained an incredibly close watch over East German citizens, to the point where virtually every residential building, place of employment or place of leisure was home to at least one Stasi informant.
The national constitution with perhaps the best protections against the development of a police state is that of the United States. Specifically, the Bill of Rights asserts those accused of crimes have certain rights, such as habeas corpus, probable cause, the requirement of specific warrants and that police actions must start with the "oath or affirmation" of a civilian complainant (which together make a prohibition against the execution of standing orders), trial by jury, the right to counsel, etc.
It is thus ironic that in many ways the United States is becoming more like a police state, especially since the September 11, 2001 Terrorist Attack, and the passing of the USA PATRIOT Act. The government now has expanded powers to surveil and detain people it considers potential terrorists without affording them rights of due process, such as those listed in the Fourth, Fifth, and Sixth Amendments. A proposed new act, the Domestic Security Enhancement Act of 2003 is likely to accelerate this process, and is being fought by civil libertarian groups like the ACLU.