You never can do too much planning for a focus group.
Manage the recruitment process actively to be sure to get the right people in the groups.
Don’t prejudge the participants based on physical appearance.
The best focus group moderators bring objectivity and expertise in the process to a project.
One of the most important services a moderator can provide is a fast report turnaround.
The most valuable service a moderator can provide is objective conclusions based on the interpretation of the research.
Achieving research objectives does not guarantee a successful focus group project.

For a detailed explanation of the tips see:

10 tips for running successful focus groups by Tom Greenbaum of Groups Plus, Inc.(yao)