Before
Set realistic goals. The senior person at the off-site should determine what the off-site is meant to accomplish then create an agenda that guides everyone toward that goal. To ensure that everyone can make an intelligent contribution you must make sure that everyone attending has the relevant data ahead of time.During
Set ground rules. Consider setting specific times where phones and laptops are not allowed. Any topics that are raised that take away from the off-site goal should be written down - "parked" for later discussion. If questions are asked of your attendees then make them open-ended questions to encourage more participation.
After
End with a plan of action. One way of doing this is to assign a champion to each action item that comes out of the off-site. The champion doesn't have to do the work but they are responsible for getting the right people started on it.