Have you ever been to one of those brainstorming meetings and felt cheated? Cheated because it wasn't really a brainstorming, it was a way of trying to get you to buy in to the already made decision!
Being a manager, being a leader means being authentic. If you have made the decision...then be honest. Describe why you have made the decision and then ask for feedback, ideas of how to implement, risks, opportunities.
If you have not made a decision then brainstorm....don't allow any ideas to be dismissed. Register any idea, and look for the kernal of greatness in all ideas. Be a angel advocate rather than a devils advocate.....
People may not like it if you have made the decision but they will respect your honesty. If you fake the brainstorming, people will see through it and will feel cheated and less likely to accept your decision.