In my experience, keeping the time is important for two reasons: expectations and hard constraints. You don't want participants to get impatient because an activity (and, by extrapolation, the whole meeting) is taking longer and longer, so that it seems there is no end in sight. You also don't want a participant to suddenly have to leave in the midst of action because he/she has another appointment.
On the other hand, you also don't want to follow the clock slavishly. You don't want to stop the flow of a productive discussion just because some arbitrary amount of time has passed. You want to focus on the people and their interactions instead of your clock.
Finding a balance between those forces that I'm comfortable with took me some time. Here are a couple of tricks and techniques I use:
Keeping the time is a service to the attendeesThis is more like a principle that informs all the other tricks and techniques: the reason to keep the time is to ensure that the participants can have the best meeting possible given the time constrains they live in. The goal is to make the best possible use of a constraint resource.
Clarify expectations and constraintsAt the beginning of a meeting, it should be obligatory to restate the scheduled timeframe. Additionally, I like to ask the participants what constraints they have on extending the meeting. It's vitally important to know whether someone has to be in another meeting at the scheduled end of the current one, whether people are eager to get home for the weekend, or whether they are totally fine with taking an hour longer than originally anticipated. Also, sometimes some people are fine with not attending the meeting to the end. All of this information has a big impact on how time needs to be handled by me as the facilitator.
Timebox activitiesKeeping a meeting inside its timebox is much easier if you timebox activities, too. Even open discussions can easily be timeboxed: "let's talk about this topic for ten minutes, and then see how we continue the meeting."
Use a simple timerUse a simple timer for timeboxed activities. The timer should be easy to read for the facilitator and show the time remaining for the activity. I prefer silent timers (no alarm when the time is over, see below) with an easy user interface. There is nothing more annoying to me than having to fiddle with a complicated timer setting at the start of an activity.
Use curt timeboxes, enforce them gentlyDon't stop the activity immediately when the timebox is over (unless it has come to a natural end, anyway). Wait for a good moment to intervene; keep the constraints of the group in mind. Using short timeboxes gives you some slack to work with.
Let the group decideIf you feel that the group might want to continue the activity, ask them whether they want an extension. Remind them of the consequences for the rest of the meeting. Sometimes the best the group can do is change the goal of the meeting to be able to spend more time on an important issue that surfaced.
Use a parking lotKeep an area on a flipchart or whiteboard where you can post topics that attendees want to discuss, but which are off topic for the current phase of the meeting. As part of closing the meeting, decide for each topic what to do about it. (I got this tip from the book "Collaboration Explained", which I highly recommend.)
Incorporate breaksA meeting that takes longer than an hour should include at least a five minute break (more and longer breaks for longer meetings, of course). Get everyone moving and open the windows to get fresh air in. A short break refocuses everyone and actually generally shortens meetings.
Plan for alternativesWhen you plan activities, for example for a retrospective, it can be a good idea to think of alternatives to long activities, that you can use instead when time runs out. This allows me to react more flexibly and relaxed when something takes longer than planned.
Blowing the timebox isn't a failureAs long as the attendees agreed on spending more time in the meeting than planned, it's a good thing that you reacted flexibly to the needs of the group. Keeping the time doesn't mean rigorously enforcing the original plan, but keeping everyone aware of time issues, and deciding collaboratively and responsibly based on needs.
That's what's coming to my mind right now. What are your tips and tricks when it comes to keeping time?