The most critical rule for any brainstorm is this: postpone judgment. Make a clear distinction between diverging and converging. Or in plain English: first come up with a lot of ideas (silly, serious, stupid, whatever), then select the best one. But don’t try to select the best one while coming up with the idea.
Why? Because it gives breathing room for new ideas. If you feel you are not judged for what you are thinking, you allow yourself to think more freely. The silly ideas become stepping stones for serious ideas. An add-on bonus is the great atmosphere you get when every person joining in the brainstorm speaks freely.
Being a critic is easy. So leave the easy part for the end.