Be Nice. Contrary to the cliché, genuinely nice guys most often finish first or very near it. I admit it's not easy when you've got a gripe. To be agreeable while disagreeing — that's an art.
Be natural — write the way you talk.
"How to Write a Business Letter" —an advertisement for International Paper Company (1979)
Being right half the time beats being half-right all the time.
As quoted in Clean Your House & Everything In It (1982) by Eugenia Chapman and Jill C. Major, p. 100