Security Auditing / Assesments

[Chad]

I am looking for a good reference or some 
established guidelines for performing a professional 
security audit. I have been a SA for several years 
now, and I know a thing or two about this, however I 
would be a fool to claim to know it all. I just want to be 
thorough, and do a good evaluation. And maybe learn 
a thing or two in the process. All suggestion will be 
greatly appreciated. This would be concerning a 
corporate environment, not a small home set up. 
Thanks in advance.