It's a collaborative effort One good idea can grow another and an idea suggested by one person can trigger a better one from another person. This kind of development can lead to out-of-the-box thinking and fantastic creativity.
Reverse the principle of quality over quantity The more ideas that get bounced around, the more likely you are to get a gem. In fact, have a competition to see who can come with the most.
Make sure each idea is recorded quickly It's easy to forget good suggestions when so many thoughts are being shared, so make sure somebody who can write quickly is at hand to jot everything down.
Keep focused on the goal of the session This doesn't mean slapping down thoughts that don't seem immediately relevant, but remember to relate all the ideas to the goal to weigh up their relevance.