When all my letters had finally been written I wrote a personal thank you note for each person expressing my gratitude for them writing a letter and for our past relationship whatever it may have been. It came out to about 8-10 sentences. At each step I let everyone know what is going on. So, when I got my first secondary I sent out a quick email, first interview = quick email. First acceptance, if I get one, will warrant a tad longer email.
Good Luck