This is actually a complex question. Just to break the ice, I think that an obvious measure is to improve the human capital level of the occupied population. Without a doubt, other colleagues will give you additional ideas.

Building slums is the response of the poor to the lack of affordable housing. As governments cannot afford to provide affordable housing sufficient to meet the need, the answer is not to prevent slums from forming, but to ensure that the people living there have access to public services, including education and health as well as roads, power, water, sanitation, and telecommunications and decent policing, that local control and governance are a reality and do not depend on criminal gangs, that residents' property rights are respected and formalized over time, and, as an overall policy, that the areas and the residents themselves are fully incorporated into the city over time. Do you think the people who built the tenements on the East Side of New York or the slums of East London had building permits?

I fully agree with Sribas that it important to begin with a thorough assessment of existing informal areas and where the deficits in services and infrastructure are greatest. This is only the beginning, however, as the main challenge is how to provide essential government services in areas where the government lacks land to put them on, retrofitting of sewerage is extremely expensive, and the funds available are woefully out of line with the huge accumulated services deficit. Of course, if governments would stop subsidizing grandiose compound housing schemes for the upper-middle-class, there would be more funding to meet the needs of the poor.

I am asking myself this same question. Slums (fallen neighborhoods as I see them) are perhaps the leftover left behind places which can hide gold beneath the feet.  Removal of old buildings and land clearing to make available tax-defaulted therefore publicly owned land is sensible. One of our past Presidents was all about this, Hamilton I think.  In examining this question from a research perspective, there is some initiatives worth looking into.  I like the 2013 Blue Green Alliance's Good Jobs Green Jobs Ohio Greenhouse example.  A brownfield was transformed into a large greenhouse lettuce business and employs local residents exclusively.  For highly concentrated population areas, rooftops have become gardens.  The government role in Ohio was some hearings, hazardous waste removal issues, permits, tax incentives, city water/sewer changes, and not sure if more was done.  I'm sorry I don't have all the details.  The real estate owner may have owed taxes in arrears.  The initial interest must be shown by local individuals before government can act in my opinion. Then it is a monumental struggle thereafter as I heard from the Project's leader during her 2013 presentation in D.C.