I have found that an important part of any planning session is the preparation. The first step is to set an agenda and decide on participants. The people who attend the planning session are often obvious but careful consideration must be given to ensure a productive session. I recall one strategic planning session I was running. I told the CEO that those people who are key to the organization in the next five years should attend. Thinking about that led him to make some management changes before the session.
I find often the setting agenda will require a short preparation meeting with some or all the participants. The agenda should be as specific as possible and organized around issues which will create a high degree of focus. Very general agenda items will lead to a very unfocused discussion and frustration on the part of the participants.
Another part of preparation is pre-work. At the agenda setting session, background work is often assigned to individuals who have the biggest stake in an issue. IF there are different positions on an issue, more people can be assigned. A one or two page position paper describing the issue and any any other relevant facts is prepared. These papers are distributed before the planning session and save much time during the session. The person who prepared the paper is given a few minutes at the beginning of the discussion of that issue to bring the group up to speed. Sometimes the person doing the pre-work is asked to lead the discussion on that topic.
All these elements that take place before the planning session can really increase the productivity of the planning session.