The InnovationTools newsletter has a fun article from Paul Sloane: Six great ways to ruin a brainstorming session

1. Having no clear objectives
   
2. Too homogenous of a group
   
3. Letting the boss act as facilitator
   
4. Allowing early criticism
   
5. Settling for a few ideas
   
6. No closure or follow through

The full article has great thoughts about overcoming these far too familiar problems.