In the follow-up, see the specific plans of the company that you discussed during the interview. Suggest how your experience and skills can help them overcome their challenges. Say that you're willing to provide the hiring manager with any additional information if needed and confirm when the final decision will be made. Choose the next logical person and send them an email.
If you've previously sent an email to a human resources person, try the hiring manager or someone in the department where you interviewed. Or the other way around: if you've been emailing the hiring manager before the interview and he hasn't said anything, try contacting Human Resources. From the job application to the interview questions to the decision-making process, the job search is exhausting. Just as the employer decides if the job seeker is the right fit for the position, the job seeker should use the interview to learn more about the company and the position and to see if the company is the right fit for him.