Writing a rejection letter in response to a pitch is rarely a pleasant experience. As Jean Hannah Edelstein says in a post on the Guardian's Books Blog, "[I]t is not easy to achieve and balance the two central goals of a truly accomplished rejection letter: trying not to make the writer feel distraught whilst also discouraging him or her from ever contacting you ever again."
Edelstien's comments come in response to the call for submissions of rejection letters for a new book, "Other People's Rejection Letters"(perhaps you have your own to contribute). While Edelstein points out that the purpose of the book is likely to provide comfort to rejectees (of every sort, not just writers), it has potential to be an excellent teaching tool for editors. I suspect it will be filled with the most crass, ill-thought-out letters out there, essentially making it a what-not-to-do guide.
I'm curious, though: rejection letters were never covered in any of my schooling; have you ever been coached on how to craft one?