Only send an e-mail to admissions if you truly want an answer to some question that's not answered on the web site. Don't just e-mail them to keep your name in front of them. They are busy people, and those kind of e-mails waste their time.
Sending a handwritten thank-you note after an interview is a good thing to do. It's also good to update them on new achievements after you've sent in your application.
If you're applying to places like HPY, they have so many excellent applicants that demonstrated interest doesn't matter.