Some colleges want lots of contacts, while others - usually the most highly selective - might interpret frequent contact as annoying. The rule of thumb is that contact with a college should have a purpose. In other words, if you contact a college - especially an admission officer - you should have a reason for being in touch and not just to keep up friendly banter.
Here are some guidelines to keep in mind:
1) Respond to communications initiated by the college, if invited to do so. For example, if you receive a postcard that requests a reply, do so.
2) Open e-mail messages and follow the links. Colleges that are interested in assessing your interest will track if you open their e-mails, when you open the e-mail, and if you follow the embedded links.
3) If you initiate contact with a college admission office, keep in mind that this is a professional relationship. This means that you should address the admission officer professionally, using a title: "Dear Dean Smith," or "Dear Ms. Jones." You should never use a first name, regardless of how friendly you feel with the admission officer.