• the solution to your problem is clear
• everybody involved agrees on what to do
• either the problem affects a simple system, or the solution must be implemented by a small team, or both
• nobody on the implementation team has authority to do anything other than what management has pre-approved.

Even in the case of complex system problems, people can be tempted to take a limited view of possible solution methods. The team members should agree to this in advance or avoid brainstorming. Remember: the brainstorming process is brief, and there's no time for distractions. Bad ideas may be a distraction, but discussing or criticizing ideas is worse. After the brainstorming period is over, there'll be time enough for

• eliminating irrelevant ideas
• grouping similar or related ideas
• commenting and modifying incomplete ideas
• ranking whatever ideas remain

...but that's a separate process. In the meantime, establish ground rules before you start:

• The group has one leader.
• The leader's only purpose is to keep things moving.
• The problem to be addressed is clearly understood by the entire group.
• Everyone has a chance to contribute. You can go around the table to solicit ideas, but this can pressure those who don't feel comfortable speaking up; on the other hand, if ideas are taken freestyle, vocal members of the group can dominate.
• Every contribution should be in that person's own words.
• No idea is criticized.
• The group has one recorder. You can have the recorder write everything on a board or flipchart, which would be easy for all to read; on the other hand, collecting Post-It Notes from each member of the group is easier on the recorder. Either way, it's important for the ideas to be in one place, where they can lead to more ideas.

References

"The Memory Jogger." Methuen, MA: Goal/QPC, 1988.
Brainstorming 101 shareware
http://www.brainstorming.co.uk/ -- excellent reference tools