#1 NO JUDGEMENT
To be effective, brainstorming sessions need to take place in a judgment-free environment. Encourage all ideas - good, bad and ugly. Have fun with it, and see where the ideas take you. If you or your team is passing judgment on the ideas of others, it will stifle creativity and lead to team members shutting down and watching the time clock for the misery to end. This is not good brainstorming. Don't do this.