The main key to a successful brainstormng session is atmosphere. Before you start thinking about techniques, spend some time thinking about the session's environment and context. What can you do to ensure the participants are comfortable working together, know enough about the issues that will come up so they can contribute, feel able to speak in front of the group etc?

For the session itself, stick to the rules: accept everything at the beginning, don't judge or re-interpret too much, and above all give people time. A short silence is fine; if you get a long pause, you can make it work by saying, 'Right, I'll give you a minute to think about that before we go on.' Have several pair and smaller group phases too, so people have a chance to develop their ideas in a quiet way before trying them out on the whole group. Oh, and if the ideas aren't coming let people have a few minutes to write down keywords or whatever on paper - it jogs the brain wonderfully.

You can find more ideas here.