The Wall Street Journal has a piece by sociologist James Wilson in which he contemplates the declining crime rates across the country and possible reasons for the phenomenon.
Since the 1960s it's been conventional wisdom that high crime rates, particularly among African Americans, were directly correlated to joblessness and poverty. Thus it was expected that when we entered tough economic times crime would once again increase. Wilson argues that this view simply doesn't fit the data - crime is actually decreasing, even during the current recession.
Some of the reasons will not be welcome among liberals, others will make conservatives uncomfortable, but they're all compelling. They include higher and longer incarceration rates, better policing (including emphasis on maintaining a presence in high crime areas), better self-protection measures available to citizens, a decrease in the popularity of hard drug use, lower lead levels in the environment, and abortion.
For an explanation of how each of these has helped to reduce criminal behavior read the full article. It's pretty interesting.