I know it might be a little late to think about this for you, but it all starts with buying houses in the right areas of town. If you buy in the rough areas, you'll be assured to draw people that thrive in those types of areas, rough people. Try to buy in areas with good school districts, higher property values and low crime rates. Any good realtor should be able to help you.
The best thing you can do now is either hire a professional mgt company to manage it for you or do as the other poster said, and really screen them before you let them in.