Bentley University
Excellent resource for citing information that you obtained from databases such as Passport, Mintel, Simmons, IBIS etc.
Purdue OWL APA Guide
Probably the most comprehensive and most used resource for citing resources. This page will bring you to the APA style section, but it contains most widely used styles by U.S. Universities.
APA Style Guide Website
Official APA page...if you care to find out more about the format.