I think that may fall more under personal correspondence. I am definitely not a etiquette guru so wait for another reply or two. I have referenced a book they gave me on I-day more than once.
http://www.amazon.com/Service-Etiquette-Fourth-Oretha-Swartz/dp/0870216201
basically it says write it within a week after you have received a gift or favor. If you were a guest at a house, address it to the hostess but make sure to mention the host when appropriate.
(date)
Dear Mrs. Doe,
1. Thank you for...
2. A sincere comment concerning the occasion, an expression of appreciation, a comment concerning mutual interest.
3. Looking forward to you again sentence
4. a thanks again and request to be remembered to the host and/or any other members of the family.
Sincerely,
Pretty common sense but I am sure you can tailor it to your needs.