The most important thing to plan for is the people who will be participating. You need to think about who will be there and know as much as possible about them. Know what they do for their jobs, what experiences have they had that you can draw from, how likely they are to be expressive and take a risk in experimenting with bodystorming.
The second thing is preparing the problem statement. This is what you should spend most of your time on and many times preparing the problem statement takes 1/2 of my total preparation time. I often adjust if not outright change the bodystorming topic several times before the bodystorming session.
The third thing is the space you will be bodystorming in. You need to know it and to have visited there.