Try PC exercises, the same type used by women to tighten vaginal muscles. Isolate the muscles by squeezing your anus tight as if holding off an 'accident', and repeat. Over and over, every day. Hold the clench for ten seconds, and relax. At the end of the day, the sphincter muscle is just that - a muscle. It can be trained and repaired to some extent, depending on how much you've stretched it out.