From an article I contributed to in Fast Company, here are four simple things you can do to overcome the afternoon blahs in a brainstorming session:
1. Meet in an unusual place. A change from your usual meeting location will help keep people awake and help force a change of perspective on your ideas and challenges.
2. Stand and deliver. To boost the oxygen to the brains, speed up the meeting and keep people awake, remove the chairs!
3. Liven up the joint. Bringing in fun props and external stimulus, watching a quick funny video or participating in a theater improv exercise will energize people and keep the meeting from losing steam.
4. Get people thinking before the meeting. Distributing a list of targeted questions before the meeting will help ensure people are primed and involved.