Brainstorming, was originally created by Alexander Osborn in 1939 (CEO of BBDO, an advertising firm in New York) to separate ideation from evaluation. There has been a lot of discussion about whether it works. My experience is that Brainstorming works when it's executed properly. Most of the complaints I read about Brainstorming describe poorly run brainstorming sessions. Follow the instructions below and you will be successful. Vary how you generate the list of ideas - calling out ideas and writing them on flip charts, individually anonymously writing ideas onto large Post-its, or listing in small groups, etc. The basics are the same, once you understand how it's done. Note: There is no such thing as a "Brainstorming Session" - it is meaningless. You use brainstorming to generate ideas about... - it is a means to an end, not an end itself. 
Note: I never use the word "Brainstorming", I just explain the process.
Purpose:
This is a basic ideation process that is designed to generate lots of ideas in a short amount of time and then analyze the list to develop a final list of acceptable ideas. It is not an end result. It is a means to an end. To succeed, you must clearly understand the end result (plan ahead) and be able to produce it without Brainstorming - by discussion instead. Note: Do not use Brainstorming to develop definitions or numerical solutions, such as, "How much money will we spend?" Author: Alexander Osborn created Brainstorming in 1939 and published books about it in the 1950's as a means of ideation.
Objectives:
- Generate lots of ideas in a short amount of time.
- Analyze the list of ideas to develop a final list of acceptable ideas.
Do the following:
Brainstorming is a 3-step process: Think (to quietly jot down ideas), List (diverge - i.e., to generate lots of ideas), and Analyze (converge - i.e., to narrow it down to a final list of acceptable ideas). To set the stage, clearly explain the process and what will be done with the results.
Think: Ask a clear, simple question, e.g., "What are your objectives?" Give the Participants 5 minutes to think about the question and have them quietly jot down their ideas on a pad of paper.
List (diverge): Ensure that the Participants can see the question at all times. Then ask them to call out their ideas, without constraints. Write their ideas onto flip chart sheets that you had previously labeled - e.g., "Objectives". Note: List all the ideas the group calls out. Stick to the Brainstorming Rules (see below) - moving away from the Brainstorming Rules causes a group to lose energy and valuable ideas. It is best to use flip charts to capture and manipulate an unconstrained list of ideas. Don't tear off flip chart sheets while listing - tear them off at the end - otherwise it interrupts the process.
When you List, use the following Brainstorming Rules:
- All ideas are allowed.
- No questions.
- No judging.
- Fast pace.
- No censoring.
- High energy.
- No discussion.
Analyze (converge): Analysis is done using a form of questions, as follows:
- "Does this belong...?" This organizes and narrows the list.
- "Can you describe this...?" Once the list is organized, describe each list item. If it cannot be described, it is removed from the list.
- Which is most important? Use a Prioritizing process to select one amongst many.
Note: NEVER ASK THE GROUP "HOW SHOULD WE CATEGORIZE THE LIST?" If you do not know, you need to prepare more so that you can provide the analysis process.
Debriefing Questions:
- What did you learn about the topic that you didn't know before?
- Are there any surprises? What are they?