The day after my wedding as we made our way from Long Island to Brooklyn, I called each one of my vendors to tell them how lovely everything was and thanked them for their beautiful work. I think I sent a note to each of them along with an issue of Martha Stewart Weddings that featured our wedding (Fall 2004).
Not sure what to write? Keep it simple. Write it to the person you worked with and mention how happy you were with their work, that you and your fiance loved everything and that your wedding was a wonderful day.
Remember it's the thought that counts.