Brainstorming is big at most organizations today, but in becoming ubiquitous it has lost something. The invitation "Let's brainstorm about that" typically leads to a gathering in a conference room where the convener asks for ideas then shoots them down as fast as they come up. Brainstorming has been compressed and made more efficient--killing its real purpose in the process.
The whole point of brainstorming is to let creativity shine. You need to be very careful not to let criticism stifle that creativity. The creative process must be supported, nurtured and embraced wholeheartedly to generate good results. To that end, I've developed 10 commandments of brainstorming, universal rules to help establish a framework for imagination and creativity.
Print out a copy of these rules and tape them to the wall before any brainstorming session. I'd also suggest you bring a bell, kazoo, drum or other noisemaker to the session. Every time someone breaks one of the commandments, ring the bell or beat the drum. Make your whole team responsible for enforcing the rules and holding everyone else accountable. Think of the rules as non-negotiable and make sure everyone on your team agrees to them before any idea generation begins.