Social Engineering Vulnerability Evaluation and Recommendation (SEVER) Project
This is a formal methodology for social engineering. There is a lot of cool stuff packed into the instructions so you must read both of the documents.
While it is largely intended for penetration testing some of the techniques are too extreme for that purpose. I want to include all of the concepts that I can think of so feel free to write a watered-down version for business purposes if you'd like.
Thank you to Andrew Reyes for the Spanish translation! SEVER Instructions Older Spanish version of the instructions SEVER Worksheet
Stupid Google Tricks. 2600 - The Hacker Quarterly. Spring 2002: 10-11
Warspying. 2600 - The Hacker Quarterly. Winter 2002-2003: 9