Job of a Focus Group arbitrator/moderator demands proficiency. You must know what it takes to start a discussion. You must have excellent people skills, ability to derive edifying responses from different people which also requires exceptional listening skills.
It is always beneficial to have a plan to decide what questions to ask and at what point of discussion the question needs to be asked. This will help keeping the discussion in the right direction.
Establish a set of laws that would help keep the discussion disciplined. This includes review of focus group duration, topics to be covered in the session, asking to turn off the phones, making aware that the session will be recorded, etc.
As an arbitrator, one should be able to break the ice, which will help participants to be at ease with each other. Remain neutral so that you may not end up asking questions that may impact participant’s answers.
Try to ask more open ended questions which would help to make the discussion more clear and must not presume what any participant means.
Apart from the mentioned points also try to include light activities to retain interest & liveliness among the participants. Most important, provide objective explanation of the research discussion.