Article from Fast Company on Six Surefire Ways to Kill a Brainstorm. A few snippets;

The six strategies below are absolute no-no's -- surefire innovation killers from Tom Kelley, general manger of Ideo Product Development.

1. Let the boss speak first.

Nothing kills a brainstorming session like a dominating CEO or the brownnosers who rush to agree with his every statement. Ideo recommends that bosses lock themselves out of idea-generation sessions all together. Send him out for doughnuts, and you'll get better results.

2. Give everybody a turn.

Kelley remembers packing 16 people into a room for one particular meeting. Each person had two minutes to speak. It was democratic. It was painful. It was pointless. It was a performance, not a brainstorm. "In a real brainstorm, the focus should never be on just one person," Kelley says.

3. Ask the experts only.

4. Go off-site.
By conducting off-site brainstorming sessions, you only reinforce the concept that great ideas only come on the beach or at high altitudes -- not in the proximity of your daily work.

5. No silly stuff.

6. Write down everything.

Luckily there are also Seven secrets to good brainstorming