Clustering or mapping can help you become aware of different ways to think about a subject. To do a cluster or "mind map," write your general subject down in the middle of a piece of paper. Then, using the whole sheet of paper, rapidly jot down ideas related to that subject. If an idea spawns other ideas, link them together using lines and circles to form a cluster of ideas. The whole purpose here is to use lines and circles to show visually how your ideas relate to one another and to the main subject.
A cluster or map combines the two stages of brainstorming (recording ideas and then grouping them) into one. It also allows you to see, at a glance, the aspects of the subject about which you have the most to say, so it can help you choose how to focus a broad subject for writing. For example, the writer of the map above his or her writing on time devices, leisure time, warps in which time passes, child vs. adult time or time in sports, any of which would provide a logical focus for an essay.