Brainstorming is like sex. It's better exercise and usually more fun when you aren't into it all by yourself. And here's where the metaphor fails me, because in brainstorming? The more the merrier - to a point. And I can't speak to sex on that count.
Like good sex, brainstorming requires some prep work. Foreplay, if you will. A bit of plotting has to have already happened. Because I'm character driven, I need to have some notion of what the character arc is going to look like for a story. That's the point at which I want ideas from outside the limits of my skull to shake up my thinking about the story. I want to be challenged and made to step outside of the lines I'd been coloring so nicely inside of. This means seeking out people who think in completely different ways than I do.
The risk is that you'll wind up brainstorming with someone who gets stuck on something. When you're generating ideas, you don't want anyone getting fixated or invested in any one notion at any point in the proceedings. Yeah. I'm not touching the sexual analogy on this one. You're on your own imagining someone getting stuck in a rut in that regard. O_o You want imagination. You want a certain mental flexibility - a willingness to throw out ideas no matter how alarming or how silly because it's all in the interest of trying new things and having fun, right? Well, and writing a great story, too. Eventually. It's not just an exercise for before a story starts. Brainstorming breaks up blocks and stuck points. Sometimes, it's useful after a draft is complete and you're making sure all of the story threads have been tied up nice and neat.
I like for brainstorming to change my perspective. Done well, it forces me to examine my story with new eyes. I can either defend my plot based on why I'm making the choices I am - or I can't. And if I can't, then I know the story hasn't yet baked. It's still squishy in the middle. Brainstorming at it's best breaks me out of the path of least resistance. While life might be best lived that way, stories rarely are the better for an author taking it easy. It's at that point that with a set of great brainstorming partners that brainstorming can become plotting - when the flow of ideas and jokes generates something that makes everyone sit up and go, "Oooo!" That's when you know. And that's when it's time to get writing.