Provide a clear focus for the session by briefing the team with the goal of the session and stating the question for the brainstorm. As already mentioned in 0. Preparation, the question often takes the form of "How might we ... ?". Make sure everybody understands the question and its relevance.
Mindmap
If the problem or question is not concrete or clear enough and consists of many different aspects, it can be helpful to start with creating a mindmap. Put the most essential word in the problem or question in the center of a large sheet of paper. Map aspects that have to do with this central word around it and draw in arrows to connect them. Keep it somewhere easy to refer to during the brainstorm session.