Rules of brainstorming

1. Quantity over quality. If you think of something, you have to say it, no matter how silly you think it is. The best ideas often start out as silly ones; the point of brainstorming is to push your brain to think of things you normally might not consider. Participate!
2. No judging. When someone presents an idea, you're not allowed to state your opinion of it during the brainstorm - you can't even say it's good or bad or that you like it or you disagree. We're trying to get ideas on the table; we'll decide whether they're any good later.
3. Build on other people's ideas. Listen to what others are saying; when you can, try to build on their ideas so that they become more specific, instead of always starting with a new idea of your own. If someone says "We should get a pony-eating-dragon!", you could say "We should build a purple boat!" - but you'll get even better momentum if you say "Make tofu ponies for the vegan dragons!" or something else related to the previous idea.
4. Use "yes, and" over "but". Instead of passing judgment, build on other people's ideas to nudge things in the direction that you want to go. Instead of saying "But that has too many ponies!" say "Yes, and we can put a pony-eating-dragon in it to keep the pony numbers down!"
5. Ideas belong to everyone. Every person and idea in a brainstorm has equal worth, and every person owns the output of the brainstorm equally - there's no such thing as "his idea" or "her idea." They're all our ideas.