You are able to contact programs about every 2 months... The NIH Academy, an abstract presentation or acceptance, a manuscript acceptance, and other things are reasonable excuses for this brief communication with ALL the programs that haven't rejected you. I suggest to make it brief with no more than 2-3 paragraphs. Indicate the reason for update, and then indicate your interest for that particular program including, perhaps the PhD that you intend to pursue and names of 4-8 faculty that you are interested in working with. Keep it short for greatest impact. For manuscripts & abstracts include the names of the submission with your name in bold and underlined, the title, and the journal or annual meeting/conference.