It is important to be well organized on the day of a focus group.
Remember, the key to a good focus group is a good group dynamic. If a potential participant seems disruptive or problematic, you should not hesitate to kindly thank them for their willingness to participate, but let them know that you will not be needing their services (just tell them that you over-recruited and have enough people). If you are providing an incentive, give this to them regardless of whether or not they actually took part in the focus group.
After that, the focus group is largely in the hands of the moderator. Once again, the importance of this position cannot be overstated.
Even if the group is being videotaped and transcribed, someone other than the moderator should be watching and taking notes on the group. Are there any important issues that came up? Great quotes? You might also consider using a flip-chart or whiteboard ask a way to jot down some of the respondents' points in front of the group so that they can all see.
Perhaps most importantly, the moderator must watch the clock! Plan how much time you would like to spend on each section of your moderator guide before the focus group begins, and make sure to keep on task when the group is being conducted.