Just like in the movie, once a year there should be a 12 hour period where committing all crimes is legal (including murder). All emergency services would also be disabled in this period. There should be a limit to what kind of weapons people could use however (no weapons like RPGs, etc). People that hold office would also be completely immune from all harm as to not create complete anarchy.
The movie made a good point in saying that humans are a violent people and that the purge is a way once a year to "purge" yourself of your frustrations and rage. It would decrease the crime rate because people would have a way to legally vent themselves. Unemployment would also go down to a low percentage because of the increased demand for home security and the deaths of people would create openings