Be open to every idea. List everything you think and don't censor yourself or others. Brainstorming starts by creating all kind of ideas that can be really crazy and by being open to all kind of ideas instead of right away rejecting some your brain become more creative.
It's important to think the place and time for brainstorming. Creative location might inspire people more than some office. It's also important to make sure people feel comfortable so that they aren't hungry, it's not too warm, or the chairs aren't uncomfortable.
Often talking with other people is better than thinking alone because as you explain ideas you might see something new or the other people might add something to your ideas. If brainstorming is done alone it often helps to say things out loud. Writing things done is anyways necessary to document ideas but it might help to understand things better.