One of the best ways to get out of a conversation involves breaking the other person's momentum - as soon as you can, ask three close ended questions. For example, if the person is talking about their favorite dog. Ask 'How old is the dog?', 'Do they like cats?', 'Is it 730 already?' - then excuse yourself politely. It's not the most tactful way to extricate oneself from a running-over conversational hog but it works effectively.