How to politely turn down a boss who asks you to go out for a drink after work?

Plead a prior commitment! For example, your folks/friends of yours/in laws/etc. have already invited you out for dinner/an evening out/an anniversary party/etc. Every subsequent time the boss asks you for drinks, play around on this variation/make up more variations. Maybe say sorry, you would have liked to, but......(insert excuse).... Eventually your boss should get the message. Hey, can you help it if you have a very active social life outside work? :-) Good luck!