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Look at things from the reader's point of view.
Emphasize reader benefit.
Remain positive.
Protect the reader's ego.
Express appreciation (not annoyance with) the reader(s).
Understand human nature. (How is the reader going to respond?)
Avoid condescending tones. Specifically, commands with "you" in them.
Don't "talk down" to your reader. Respect his/her intelligence.