First, as we’ve said many times before, you should ALWAYS send a thank you note after a job interview. Ideally, you’ll get this to the interviewer within 24 hours (but on the same day if possible!). Unless you’ve heard that your potential boss has some strange obsession with hand-written thank you notes, you should send an email -- it’s the quickest and most efficient way to get the hiring manager’s attention, and it creates a thread that makes it easy for him to follow up. If you are asked for references or sample script coverage, be sure to send that over right away with your thank you email (or by a set deadline). Otherwise, be patient, and wait for the interviewer to come back with more information.
Hopefully, you were able to get some type of answer about the hiring timeline at the end of your interview. If you were told they would be making a decision about second round interviews in two weeks, you should sit tight, and follow up a couple of days after the two week deadline, giving them a chance to get back to you first. If you didn’t get a clear picture of the hiring timeline, you’ll have to be a little more strategic in your communications. A good rule of thumb is to wait a week and a half and two weeks for each follow up email. Emailing more frequently will irritate the hiring manager, especially when they’re in the middle of interviewing tons of candidates alongside their day-to-day job. Hiring a new team member puts added stress on the department, so keep in mind that they’re trying to fit you into an already tight schedule.
If the hiring process keeps dragging on over weeks or even months, try not to get too invested in the position. Sometimes the process gets put on hold for a variety of reasons, or you simply may not be a frontrunner. At this point, your follow ups can be more infrequent -- an email every two to three weeks will show the hiring manager that you’re still interested and available but aware that these things can take time. Remain positive, and find a middle ground between being eager and overbearing so you don't burn a bridge.
If you’ve waited all this time and are finally notified that you didn’t get the position, don’t worry -- your careful follow ups have kept you on the hiring manager’s radar, and you may be able to use this contact to land a different position in the company. If your main point of contact is an HR recruiter, continue to follow up about job postings you see or ask if he knows of any opportunities that may become available soon. If you were interviewing with an employee in the department that's hiring, find a way to build a relationship with him and keep in touch. Especially if you were a finalist, you likely made a good impression, and the employee will surely be willing to sit down with you for an informational interview or maybe even meet you for drinks. It’s very common for a solid interviewee to return to the same company to interview for additional positions, so don't despair, and maintain whatever relationship you can.