I’ve grabbed this post directly off of teh US-CERT security site.  To review the original and other great articles, please visit the site at
http://www.us-cert.gov/cas/tips/ST04-014.html

National Cyber Alert System

Cyber Security Tip ST04-014

Avoiding Social Engineering and Phishing Attacks

Do not give sensitive information to anyone unless you are sure that they are indeed who they claim to be and [...]