This article is from the FAQ, by with numerous contributions by others.
Here is a topic for another book. In a nutshell:
Typically, in order to have a good chances of success, we must investigate
beforehand whether the salary being offered to the alien meets the likely U.S.
Department of Labor ("U.S. DOL" or "DOL") prevailing wage standards. We must
also carefully analyze the job opportunity to establish its actual minimum
requirements for the job (these requirements are often not identical to the
personnel department's rudimentary job description and requirements) and to
harmonize these requirements with the standards established by the DOL for
review of job duties and requirements. We might also want to investigate the
current status of the labor market for the position, to assess the
potential availability of qualified U.S. workers.