Tom Kelley from IDEO makes the point that brainstorming meetings, shouldn't resemble at all, any other type of meeting.
He's right. Brainstorming is a special activity that runs counter to the thought process and flow that happens in almost every other type of business session. Brainstorming is about quantity of ideas, freedom from constraints (mostly), lack of evaluation and critique. It's an artistic exercise more than it is a scientific one.
The best brainstorming sessions seem to be the wackiest. They include games, lots of physical movement, silliness, and dare I say...fun.
This often clashes with the rigid professional culture at many organizations. Tough. Innovation doesn't care about your corporate culture...nurture it by any means necessary.