The last 15-20 minutes should be devoted to wrapping up a brainstorm in three parallel steps. Each examines and analyses the initial ideas on the flip charts, and finds ways to transition them into something useful and tangible.
1. Clarify means using explicit and agreed-upon criteria to quickly sort the 10% good ideas from the 90% not-so-good ideas. This post explains creative criteria in more detail. This post explains the 90-10 Rule in more detail.
2. Amplify means to combine and merge the average, mediocre and half-ideas together, creative better ideas.
3. Verify means to check to ensure the ideas will do what you need them to do.