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Western Michigan University: Writing Positive Messages
Excellent and in-depth site on how to write short, positive messages. Gives information on structure, pace, positive replies, announcements, and apologies (used as positives).
Online Writing Lab at Purdue University
Purdue University Owl: Accentuating the Positives in Business Letters
Online resource from Purdue on business and technical writing. Details how to write positively through embedded position and effective use of space.