Never be afraid to share your ideas with those around you. And not just with other writers... anyone who's old enough to understand what you're talking about. Sometimes they will utter one word that sends you down a path to a great idea. In fact, my 12-year-old son just did that today, and now I have a great ending for the 2nd Bruten & Tommy book.
And never limit brainstorming to a set amount of time. Often times, good ideas come after a brainstorming session. That's primarily because your subconscious is still hard at work trying to come up with a solution. The power of your subconscious mind is not to be underestimated. "Let's sleep on it" really works!
And one last thought about brainstorming: Follow all ideas down a path before you rule them out. Sometimes a seemingly mundane idea can lead you to a fantastic idea. It's as if you've found one piece of a puzzle, and your mind pulls together the other pieces to see what the true picture will be.