When dealing with jerks threatening litigation:
"I'm sure you'll do what you think is best."
When dealing with complaining letters, Mark Twain used this one (I have also found it useful):
"Dear Sir or Madam, You may be right."
But my favorite of all time came from D.D. Eisenhower:
"Things are more like they are today then they ever have been before." Drop that one at your next meeting and watch people scratch their heads.