Ever attended a meeting about Underwater Basketweaving, only to have it devolve to trying to solve the problem of what underwater medium is the best for any kind of weaving?
Ok, probably not. But the gist is that a lot of meetings go off topic; oftentimes, the topics are way off topic: people's pets, plans for the weekend, personal crusades, love of puppets, etc. Sometimes, they wander within the realm of the topic--as noted above, underwater mediums--but don't accomplish the actual purpose of the meeting, which is Underwater Basketweaving.
In previous "Musings on Meetings" I've recommended an agenda as a method for making a good meeting. An agenda can help, but doesn't always constrain attendees. The more creative or motivated (or both) an attendee, the more often they are to go off topic. Further, their off topic remarks or thoughts are useful work...just not useful to the work of the meeting.
I have three tricks that seem to get positive results in keeping a meeting on target (if you don't count the agenda as one of them).
Option 1: Redirection
Just like kids get distracted by shiny objects and suddenly are no longer interested in walking to the park to play, adults have that happen, too. With a kid, you separate them from the item that has their attention, at least momentarily, remind them of their commitment to the original task, and redirect them there with promises of how much fun they'll have once they get there.
Works relatively similarly for adults. Stop the conversation as soon as you notice the derailment, remind them of the purpose of the gathering and what, by attending, they've agreed to discuss, and then enforce the message of the meeting by reminding them of the good that will come out of it. For example, fewer upcoming meetings, faster ability to complete work, going to lunch earlier, etc. People like to think that they make good commitments and decisions, and redirecting them back to the commitments and then validating the choice to make the commitment is a great way to get them back on track and keep them happy. No one likes to be bossed around (kids or adults) and this is a way to help them get the work done while still feeling good about the work and you.
Option 2: Alternative Options
Sometimes you can't nip it in the bud fast enough, or its really good thoughts/meaningful work, but not work that you need to do right now. In this case, I like to tell folks, "This is an interesting thought, let's spend two more minutes on it and let me get some notes down, and then I will schedule a follow up meeting." Then that's what I do--let them have 2-3 more minutes of the topic, take notes on open questions and new ideas, and then stop them and redirect them back to the meeting, reminding them you'll schedule another meeting (or talk to the person in charge of that set of topics) to schedule another meeting so that they know that it won't be lost and that they can talk about it, just not right now. Giving them a few moments to empty their brains of what they currently find exciting/interesting can often make the subsequent conversation significantly better (since they know they can work on those other problems at another time, and they've been validated in their interest in that other topic).
Option 3: Combine Option 1 and 2 and Documentation
Sometimes you need to let people talk, but you also need to get them back to topic. Combining 1 and 2 and documentation, will do that for you. After the meeting, you can remind people that the second subject will be discussed later, and what steps lead to that discussion. You can also include your notes from the meeting about both topics.
Option 4: Give Up
Seriously, give up is an option. If it's late on a Friday, the day after a late night ship, ten minutes before lunch...the list goes on of times when attention spans wander and you have to calculate the return on investment of trying to get them to focus or just let them go and reschedule the meeting. Never discount the value of giving up, but never fail to submit the next meeting invite, if you give up, the same day you give up. You don't want folks feeling like their lack of participation will dictate whether or not certain problems get discussed and solved, but you don't want them to try and solve problems if they aren't all there.
These tricks often work pretty well when you're not the one who called the meeting, as well, though I'd be sure of the strength of my relationship with the meeting organizer before horning in on the organizer's territory. Sometimes they are relieved by your help, other times, annoyed. Best to talk to them before it comes up and there are no surprises--well at least between you two--in the next meeting, which may wander to Atlantis or the future of the server farm.