Employ "I messages".
"I messages" are irrefutable statements about the communicator's feelings, thoughts, observations, perceptions, and reactions. When confronting someone, talk in terms of yourself, for example, "I think," "I feel," "In my experience," "for myself," "I need," "I've observed."
State your request clearly.
Be honest and direct. Do not over explain or apologize.
Focus on the offending behavior involved, not the person.
Be specific in your description of the behavior and the effect it had on you.
Use appropriate non-verbal behaviors.
Show the intensity of your feelings. Look the other person in the eye. Speak clearly, without hesitation, leaning toward the listener.