If you have already followed up after the interview and ...
*Ask near the conclusion of an interview as to what the next step would be, and within what time frame this step and/or a decision would be likely. Of course, along with asking this question, be sure to express clearly your interest in the position.
Often, other organizational priorities surface and the hiring process gets delayed (and some companies have a fairly lengthy hiring process). While this can be frustrating, it is often the nature of business. The good news is that a delay may have nothing to do with whether or not the employer views the candidate as the right fit; it may stem from other unrelated issues.
A second follow-up of this nature can be via letter, e-mail, or even telephone. A telephone call is a little more intrusive and may put the employer on the defensive unless you are very tactful; however, it is an option, especially for sales/marketing/public relations types of positions (where tactful tenacity is a desired skill). However you follow up at this stage, reaffirm your interest in the position and ask how the hiring process is proceeding.