It's not really hard: Avoid military coups. Implement wealth distribution and social programs, with an European model (i.e Sweden, Germany). In multiethnic countries, make movements for equality stronger, but not falling into outright communism. With a culture of stable democracy, corruption will be less endemic than OTL. And there you go.
Of course, to prevent the endless cycle of coups and counter-coups, you'll need to prevent US/Soviet involvement in Latin America. And that's easier said than done. Maybe either:
1: The USA considers it to be firmly on its' sphere, and the CIA does not feel the need to foster coups, or they intervene in the countries on a different way.
2: A stronger Non-Alligned/South American Unionist movement scares away the US and USSR, and Latin America seeks its own path.
3: Latin America is firmly in the USSR sphere, and it remains stable under communist regimes (near impossible, and probably would result in a way different Latin America)