Brainstorming works best if we can resist the temptation to judge the ideas.
Let the ideas flow and judge them later
Allowing anyone in the group to say, 'That won't work', 'That's unrealistic' as you brainstorm stops the flow and encourages the most vocal people to dominate.
Make your judgements too early and you may miss out on the opportunity to allow one idea to spark another. Most inventions are developments of other ideas: the car was a horseless carriage; the bicycle seat is still called a saddle and until recently was exclusively leather.
When you feel the ideas starting to dry up, try focusing on what you have already come up with and brainstorm ways of extending them.
Evaluate your ideas
Finally, make your judgements about which ideas or combinations are the most promising or practical.