Study says... remaining friendly from a distance is OK if you share kids. Otherwise cold turkey is the best way to move on. If you or an ex is keeping you around they may actually be a psychopath!
The only ex I maintained contact with was someone that I was good friends with before we attempted to date. He's since gotten married and has a kid so we don't keep in communication now, but when I do see him and his wife it's always pleasant. Everyone else... I have enough friends and I don't generally wish to keep an ex around.