When to make a follow up phone call is critical to an interview. First, make sure you've gotten a business card from your interviewer so you have his or her contact information. After your first interview, send the interviewer a thank you email that very evening. Next, wait around five business days, and make a follow up phone call to the interviewer. Let him or her know your continued interests in the position, and find out if there is any further information the interviewer needs from you. The key is, do not be over persistent. Be casual.
After those steps are taken, you've done your job. You've done more than 75% of the other applicants. As I stated before, the key to this procedure is to remain calm and casual, but not over persistent where the interviewer may begin thinking negative thoughts about you. Good Luck.