It's also a good idea to set a time limit of 30 to 45 minutes, depending on the scope of the session. Allow no criticism. In brainstorming, all ideas are valid. Look at the problem from many different points of view. Encourage people to build on the ideas of others.
Combinations often yield new ideas superior to the originals.
Start your session with a clearly stated objective. Write the objective in a prominent place using a whiteboard, an overhead projector or a flipchart. Pictures, toys, magazines, audio or video excite imaginations and conjure up mental pictures of creative solutions. Once you've made the objective clear, give participants four or five minutes to scribble down their thoughts. Be prepared to write down all ideas as they occur.