Anyone who's seen me speak knows that my rule of thumb when it comes to staying in touch with employers is that you stay in touch three times per year. I'm not in school anymore, but I still follow this concept of staying in touch three times per year -- fall, spring and summer. You want to get in the habit of building below-the-surface relationships. Find out what drives people. Did your boss just get married? Is your boss going to Hawaii over the summer? Is your boss obsessed with his/her dog? These are the talking points that you should bring up when you are staying in touch three times per year.
These should be quick emails -- no more than four or five sentences -- that quickly update the employer about what you have going on in your life, and act as a friendly check-in to say hello. You don't always want to contact an employer when you need something. Follow my three times per year rule, and it will lead you to career success!