“When to say No”, from the Mayo Clinic website on stress management:
Focus on what matters most. Examine your obligations and priorities before making any new commitments.
Weigh the yes-to-stress ratio. Is the new activity you’re considering a short- or long-term commitment?
Take guilt out of the equation. Don’t agree to a request you would rather decline out of guilt or obligation.
Sleep on it. Before you respond, take a day to think about the request and how it fits in with your current commitments.
How to say no.
Say no. The word no has power. Don’t be afraid to use it. Be careful about using substitutes phrases, such as “I’m not sure” or “I don’t think I can.”
Be brief. State your reason for refusing the request, but don’t go on about it. Avoid elaborate justifications or explanations.
Be honest. Don’t fabricate reasons to get out of an obligation. The truth is always the best way to turn down a friend, family member or co-worker.
Be respectful. Many good causes land at your door, and it can be tough to turn them down. Complimenting the group’s effort while saying that you can’t commit shows that you respect what they’re trying to accomplish.
Be ready to repeat. You may need to refuse a request several times before the other person accepts your response. Calmly repeat your no, with or without your original rationale, as needed.