Separate out the creative act from the act of editing and execution. Make it a two-step process. First, let ideas flow and encourage EVERY idea to make it to the whiteboard. Don't criticize, judge, edit, budget, or worry. An idea on the wall can't hurt anyone, so let them rip without restriction. After any and all ideas have the opportunity to "come out to play", only then should you apply your analytical and logical side to the effort. Don't mix the creative process with the editing process or you'll kill your ideas before they even get a fighting chance.