According to an article in today's NYTimes.com, "Most Crimes of Violence and Property Hover at 30-Year Lows." Three important things happened after 1995 -- the year in which the rate of violent crime began to drop markedly. First, the incarceration rate continued to rise: Persistence pays off. Second, the percentage of the population that is male and 20-24 years old continued to drop, in keeping with the general aging of the population. (Age usually brings with it a greater degree of maturity, stability, and aversion to committing criminal acts.) At the same time, spending on criminal justice functions (police, corrections, and courts) continued to rise, especially spending on police.
I'm sure there are other causal factors, but those are probably the big ones. The first and third of those factors -- incarceration and spending on the criminal justice system -- go hand in hand. And they are the public-policy weapons of choice in a society that values individual responsibility.