Send an initial thank-you email within 24 hours of the interview. Restate your interest in the job, and perhaps mention a point that came up in the interview that was of special interest to you. If you interview with several individuals, write each of them.
If you really want the job, send an additional hand-written note expressing your gratitude for the opportunity to interview. This is also a good opportunity to reiterate or elaborate on a particular question. Proofread for grammatical or spelling errors, including the names of the interviewers.
If you are not contacted within the specified period of time after the interview, call the employer. Politely and firmly express your continued interest in the position and concern for a decision.