Here is an example of a successful letter used by author Greg Linnell to gain permission for providing open access to a forthcoming article, to be published in a subscription-based journal. Publication details have been removed for privacy reasons, and to facilitate use of the letter as an example for other authors.
Enclosed you will find a signed copy of [your journal's] author’s contract together with a signed copy of the SPARC Canadian Author’s Addendum to Publication Agreement.
My intention is for my contribution, “insert title here,” should it be accepted, to appear on [institutional or disciplinary repository, e.g. E-LIS.] As mentioned in the Addendum, [journal title] will be mentioned as the source for the first publication of the article.
[Insert statement of your support / involvement with OA, e.g. "I have been a long-time supporter of open access and am currently a member of the Canadian Library Association’s Task Force on Open Access"]. If you wish to learn more about the Addendum please consult the following Canadian Association of Research Libraries brochure: http://www.carl-abrc.ca/projects/author/EngBrochure.pdf
Thank you for all of your hard work in getting things going again at [journal title]. I am sure that I am not the only contributor who is extremely appreciative of your efforts!
Thanks and congratulations, Greg!