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Western Michigan: Writing Persuasive Messages
A comprehensive guide to writing persuasive and sales messages and letters. Includes information on requests, sales, letters that sell ideas, and collections.
Online Writing Lab at Purdue University
Purdue University Owl: Sales Letters: Four Point Action Closing
A good resource for effective sales writing skills. Describes a four-point system of attracting and convincing customers to order the product.
Other
Copywriting Tutorial: Identifying Your Usp (Unique Selling Point)
Every successful ad has a unique selling point even if you don't think there is one. Tips on how to determine/create USP for any product or service. Very readable & puts viewpoint across in interesting & memorable manner.