DF is a HUGE talker and does this all the time no matter who's talking. When it's him and me, I've learned various tactics, but they might not be polite here so I wouldn't suggest them (they're effective though abrupt at times). When he does this to other people, I've learned to redirect the conversation to whatever the person was discussing (because I've found that most people have either been too polite or too timid to get the conversation back on topic). I'm not saying that *I* determine the conversation flow, though. I mean conversations where, for example, someone is in the middle of telling a story, and DF interrupts then goes off on a tangent then another tangent then another tangent until the original story is lost. He's done this a ton of times. I also will just interrupt him when I can see that a) the other party (or parties) is trying to exit or b) when he shows no sign of letting up (apparent when he starts up a new topic). I'll just step in and say, "We need to get going" or, if I've this level of comfort with the other party, I'll step forward, hug the person and say goodbye. Otherwise, DF will NEVER.STOP.TALKING. He said when he was four years old, his great uncle turned to his mother and said, "He NEVER.STOPS.TALKING" (DF gets it from his mother, I'm told, so I don't know if she felt he's an excessive talker).
I'll admit, though, that I do get a bit of evil glee when we encounter another excessive talker and interrupter because I figure they can just wear each other out and leave the rest of us out of it. When someone engages DF or encourages him to keep talking and I can see where the conversation length is headed, I'll sometimes wander off and do something else. I figure the other person could save him/herself if he/she wants to encourage DF to keep talking.
I did once do the putting up the hand thing to DF when we were talking to someone once. Actually, I kept trying to ask the person a question, and DF thought he (DF) was just so funny by continually interrupting me to make some lame joke. Finally after DF did this for the third time in just a few minutes, I physically turned my body towards DF, put up my hand and said, "We need to get out of here soon in order to make it to such-and-such on time, and I still need to ask Person a question so you need to stop talking so I could get my question in." THAT is what finally worked.