MIT Sloan Mgt. Rev., Vol. 49, Summer 2008 at 11, offers some good ideas about brainstorming (See my posts of Nov. 28, 2005: mind-map software helps with brainstorming sessions; Dec. 9, 2005: the Delphi technique; Oct. 30, 2006: suggestions and two other points: have rules and push participants to prepare ahead of time; and Nov. 25, 2006: more techniques to improve brainstorming.).
Researchers compared the quantity and quality of ideas generated by brainstorming groups of two kinds: one where all the effort took place in a group setting and the other where members prepared on their own before joining the group to share their thinking.
The traditional groups consistently came up with the best ideas, yet also the worst ideas. The hybrid groups “produced more ideas that were, on average, of higher quality.” Furthermore, traditional brainstorming groups are weaker when it comes to recognizing the best ideas they’ve generated.
If you want your brainstorming group to come up with lightening bolts, have members ponder on their own and then come together to produce some creative strokes.