Most people in the prison system are there for non-violent offenses. [1] In Chicago, blacks were arrested (before decriminalisation last year) for cannabis consumption at a rate of 15 persons for every 1 white person despite similar rates of cannabis consumption among blacks and whites. [2] People who have gone to prison for any of offense have a 40% chance of eventually going back to prison. [3] I would suggest that the change in incarceration rates starts with less racist policing.
[1] http://www.bjs.gov/index.cfm?ty=pbdetail&iid=1118
[2] http://m.chicagoreader.com/chicago/chicago-marijuana-arrest-...