If you are leading the process, you need to identify holes in the idea, process and people that might detract you from your ultimate goal. Counter to popular belief, this doesn’t always become easier with experience. With each new idea come new people, new processes and new hurdles for a new goal. What remains constant is to be able to say no to bad ideas which could quickly balloon to take up your precious time.
Each time you add a new feature, you make another promise to your customer. Before you do that, check if you have done all you can to keep the promises you have already made. Focus on your promises and say no to the rest.