For brainstorming to work, the participants have to take a little bit of personal risk. Everything that one says may not all make sense or is well considered - that is the point. Some people may hesitate to put forward ideas that are not perfectly formed.
If you know that there are some perfectionists who can become reticent in a blue sky brainstorming session, Brene Brown's TED talk titled "The power of vulnerability" might be just the thing to help participants relax and embrace not being right all of the time.
Another way to make it safe for brainstorming participants to contribute crazy ideas is to leverage the "yes, and" rule of improvisation. See this article on how improv can help enforce the "no idea is a bad idea" ethos that helps make brainstorming sessions productive.