Also: equally interesting is the way you end a letter. I've always liked Vellos' "Pleasant days and pleasant nights" (or something? tried looking it up, but I don't have any letters left). I always find it rude not to have anything between the signature and the body of text, certainly for foreign rulers. I can't imagine they simply sent tweets to each other like that - certainly not on Dwi 
Though I have sinned against this principle myself.
(I think it's "Long Days and Pleasant Nights" and its actually from the Triunist faith... I actually haven't paid attention to if he has still been using it since he converted. But OOC I think its from some book series or something.)
I really enjoy the sign-offs aspect as well. It is perhaps one of the most dynamic parts of letter writing and can tell you a whole lot about someone.
"Respectfully," is always good for nobles above you and especially ones you do not know. "Yours," or "Respectfully Yours," for those close friends. And then I usually always try to come up with a standard sign-off for each of my characters.
