To add to
@Catalystik, I'm always a fan of the classic three-paragraph format, usually with no more than a few sentences per paragraph. Following this format will keep things concise and cut out any extraneous information that you don't need to include in there. When I send these sorts of letters (be they thank yous or whatever), I use:
Paragraph 1: quick introduction, something positive about the school or what you've been doing, short sentence saying you'd like to thank them/update them with new information
Paragraph 2: main content; if a thank you, then try and be specific and identify things that you enjoyed about the program or the time you spent with your interviewer; if an update, provide the key information and describe what you'll be continuing to do for the remaining time until matriculation
Paragraph: end on something positive or hopeful, e.g., "I'm excited about the opportunity to come here...," "I really enjoyed my time at...," whatever.
The other thing I would add is to not be afraid to include some character in your writing. 99% of thank you notes I get are boring and seem painfully still and formal. Don't copy/paste the same stuff and send it everywhere. "Waste" the time to write the note (this is true of updates as well) each time so that you add a little character to each message. There's nothing worse than reading the most bland and boring message that doesn't actually say much beyond "hey, I sent you an e-mail!"
I know some of this doesn't apply to OP, but just some general advice.
