Toughening up! How do you deal with it?

How do other Gnetters deal with rude people i.e. friends, family, people in supermarkets, clubs etc? I am quite a peaceful person inside and, since a rather nasty falling out with relatives visiting a few months ago (difficult entitled niece), I lost my confidence when it comes to responses. What phrase do you use to check them in place immediately? I usually fume afterwards for weeks and wished I had been more assertive. I could do with a nice short phrase if anyone has any up their sleeve.