Poverty and proximity are major factors increasing and sustaining crime, and if people don't commit crimes they don't become incarcerated prisoners. So better social systems for people who need it and forcing wealthy citizens to co-locate with poor citizens so they have to share the same goods and services instead of allowing a multi class system in everything but name. And for the US obviously outlawing for profit prisons and the prison slavery clause of the 13th amendment. No crime justifies slavery as a punishment. That'll reduce prison populations by such a large amount that you could easily manage rehabilitation and humane incarceration programs for each individual.