There are several ways to follow up and you may want to let your interest level in each position guide you. Thank-you cards and letters, and strategic letters are most common.
Strategic letters are considerably more effective; however, they require a little more effort on your part. These letters can be sent via e-mail or postal mail. It is best to save thank-you cards for those who have been particularly helpful, such as administrative assistants, etc.
Strategic follow-up strengthens the interviewer's perception of you and addresses any concerns you felt the interviewer might have about this position being the right fit for you. It also provides an opportunity to add any related skills, abilities or interests, and other information that you did not think of in the interview, which may have a bearing on your candidacy. Since there will typically be several top candidates for the position, each with various tradeoffs regarding strengths and liabilities, this follow-up helps nudge that often difficult decision in your favor.
Immediately after the interview, it is essential to write down particulars. Include the details of the job description as described by the interviewer; as well as specific information regarding the company and department in which you would be working and any skills for which you felt the interviewer had a concern.
Keep in mind that while follow-up can make the difference in being extended an offer now, it also helps to leave doors open for the future; interviewing for one position may lead to another.