This means listening to them first and making them feel like you've heard what they've said. Make your argument in the form of a story. Use parables or metaphors to hook their unconcious minds. Use positive images to convince and gain agreement. Tell the truth.
Truth is power.
Tell them what you want. State your goals explicitly. Vague, wishy-washy requests are likely to be misunderstood and misinterpreted. Don't leave the others guessing about what you want. Avoid sarcasm, ridicule, attacks upon individuals or their professional credentials, or scorn.