I just read this interesting post about a scrum team that borrowed the concepts of yellow and red cards from soccer to aid their standups. 3 yellow cards or 1 red card means "move on".
Unfortunately, I live in a country where soccer is not the biggest sport... and I'm sure we would just misplace the cards.
So, let me share what has been working for my team for over a year now...
If anyone in the group feels that the current topic is not relevant to the group, they slowly raise their hand. They do it slowly to not be obtrusive and rude. If other people agree with this person, they can decide to also raise their hand. If there seems to be visible consensus, the people speaking must quickly assess their conversation and decide how to close it. They can either put it on the parking lot, or realize they aren't getting anywhere with it and just stop.
The parking lot is a list on the wipeboard in the team room. The person closest to the board writes the topic and the names of the people who are involved. At the end of the meeting, those people stay behind to pick the conversation back up.
If the person raising their hand is alone when their hand is fully raised, they suddenly realize that they are alone in their stance and drop their hand. At this point they wait for the good of the team since clearly, everyone else finds the conversation valuable.
And yes... our standup is short. We have about 15-20 people every day... and we average about 17 minutes (15-20 minutes) even with these included conversations.
Recently, we had an amusing moment when someone raised their hand on themselves...