Dear …
After giving it great consideration, I have decided to resign my position as __(name of position)__ with __(Name of organization)__. Other areas in my life, which I have already left unattended too long, are currently biding for my attention. It is time now that I answer those callings.
Please be advised that effective __(date/immediately)__, I will be leaving my __(name of position)__ with __(the type department/the board of directors/Name of Organization/etc.)__. All the projects __(I have worked on/currently in progress/etc.)__ are __(all completed/well documented/etc.)__. This should make the transition for my replacement fairly easy.
I am grateful to have had the opportunity and experience to __(act as a member on the _type_ board/have been part of your team/etc.)__. It has been both a learning and valuable experience for me. Please accept my best wishes to __(Name of organization) __ for continued success in all future __(endeavors/productions/etc.)__.
Sincerely,