Good eye contact. Easy smiles. Awake and alert. Don't hunch in your seat.
At the end of the interview, I'd look 'em in the eye and with a lively, pleasant expression, I'd say "Thanks so much for the interview. This is genuinely a dream job for me - I really appreciate you considering me for the post." And then handshakes all around. Make sure, btw, that your handshake is not a dead fish handshake nor a Paul Bunyon handshake, but something strong and steady, like you.
Then I'd follow up with a very brief hand-written note on good cardstock.