I think you should contact them every one or two months, preferably by letter. The important thing you need to do is have something meaningfull to tell them. Try building on one of your strengths, maybe take on a greater leadership role in one of your volunteer endeavors or some other progression. A letter is better because the adcom will have it on hand when they look at your application. If you call, you'll mostly just say a few words to a secretary that's not really part of the true admission decision. Good luck!😀