While doing some research on letter writing, I came across a Web site that has topics such as "The Art of Writing Letters" and "12 Rules for Writing GREAT Letters," both written by Pamela Wright.
The Web site is WrightsLaw, and it's an advocacy Web site for parents of children with disabilities. The letter writing articles all focus on writing letters of complaint or concern and attempting to remedy a bad situation.
Even if you don't have a child with disabilities or if you don't have a complaint, the letter writing tips are interesting to read. Many of the "12 Rules for Writing GREAT Letters" can be applied to most any letter writing situation. For example, the first rule is before you start writing, know why you are writing and what you hope to accomplish. This is a good rule to follow, whether or not you're seeking justice for disable child or you're simply writing a note to your best friend. Having an idea of what you want to write about makes your letter easier to read.
"The Art of Writing Letters" suggests using a story-telling approach to writing letters. Again, this works for complaint letters or personal missives.
I think it's an interesting site.