I treat it like changing trains. A quick sorry to jump in, I've gotta run to the restroom or I promised to say hi to the new guy, and I start moving while finishing the sentence. If they are mid-story, I touch my chest, say I want to hear the end but I need to handle something fast, and ask them to tell me the punchline later. The motion does most of the work. Last time I waited for a clean stop I ended up holding a plate so long I dropped hummus on the mayor.
