Timebox and Clockwatch
I carry an electronic kitchen timer with me. It fits in a pocket of my laptop bag. (I take the battery out because when it beeps in the airport it makes TSA people nervous.)
When a team and I dip into a conversation that we're not sure is a tangent, I'll say, "Let's give this five minutes and see where it goes." So I set the timer, and away we go. At the end of five minutes, the group makes the call either to continue or to park the discussion.
Finally, watch the clock on the wall. If you've expended two-thirds of your meeting time and you're nowhere near meeting your objectives, it's time to start making a backup plan or schedule a subsequent meeting. Or better yet, if you've met your objectives, finish early.
Staying on track takes practice. And having a few tricks in your toolkit doesn't hurt.