Scenario Frames
One of the best ways I've found to map out a problem space is to create what I call, "Scenario Frames." I call them scenario frames because they are an organizing frame of scenarios. Simply put, they're a set of one-liner stories or scenarios organized by categories. The beauty of a scenario frame is that it can contain macro-level or micro-level scenarios and, because it's a tickler list, it's easy to traverse.