I want to paint over an old polyurethane/stained floor. What is the best way to prep the floor?

Five years ago, I sanded, stained and polyurethaned my remodel home. I am changing one of the rooms and wish to paint the floor. The polyurethane is chipping and flaking. I know that if I paint on top of this surface it will not stay. What is the best way to prepare the surface for a smooth, durable paint job?