It is hard to attract the best job candidates with a lousy job description. Christine Lagorio-Chafkin writes for Inc. with a few quick tips to get it right. It starts with defining the position, which, if the job is not utterly orthodox, might mean assigning the actual job title after all duties have already been decided. The job description itself needs to include the job title, the department, and the position to whom the new person would report. A bulleted list of duties in concise language should follow an introductory summary paragraph. If you want to be crafty, include “duties as assigned” in the list to protect yourself if the new person has to take on duties not originally intended. And when it comes to qualifications, be as specific as you can afford, and address them in a separate section. Bring in legal HR if you want to guarantee the language being used is kosher.
For elaboration, you can read the full article here: http://www.inc.com/guides/2010/04/writing-job-description.html