RadCon as a visiting professional (i.e., a writer). I was on four panels and had to moderate two. I was warned beforehand that one people on one of the panels was long-winded and hard to keep from dominating the discussion. So I came up with a plan. This is one way to moderate a con panel (probably not the only way):
1) I wrote six questions (I had help from fellow members of my writing group).
2) I set the alert on my countdown time to the most annoying sound I could find.
3) When the panel started, I explained that would ask a question and then start the timer for seven minutes. At the end of the seven minutes, we would go on to the next question. I chose seven minutes because 7 x 6 = 42 and that would leave me 18 minutes for introductions and questions at the end (some of the panel members were so long-winded in their introductions I wanted a time then, too).
4) I would ask the question and hit the timer. When the timer went off, I would let the person finish their thought and then move on to the next question.
This seemed to work very well, although the long-winded person got annoyed at being interrupted by the timer, it did keep her from dominating the panel. There was time for questions at the end and one of the audience members approached me to say she thought my method worked very well.
It actually worked better on the panel with the long-winded person than the other panel. On the other panel they would cover about three of my questions in the first seven minutes. So I decided to let them talk longer and would pause the timer until I decided we'd beat that subject to death.
If I'm asked to moderate a panel again at a con, I will probably use this method again. It keeps the discussion moving and not stuck on one subject for too long.