The best way to prevent paint from peeling is to make sure the surfaces are properly prepared before you start painting. Remove all peeling and cracked paint using a wire brush and a paint scraper. Then grab some 120-grit sandpaper and scratch the paint from the entire surface or porch, so the new paint adheres.
If you want an even better bonding surface, paint on a layer of primer to seal the surface and cover any minor cracks. You should also get better coverage if you use primer because the paint has less work to do.