SANS paper entitled "The Art of Web Filtering". I disagree with
the author only where he states that keyword filtering is poor, at
best. My view is that it really, REALLY stinks as a tool.
depending on the size of your customer base and what you're trying to
block, filtering may become an exercise in futility as you try to keep
up with your policy abusers. It's much more efficient to have an
enforceable policy and self-policing users.
Public prosecution of
offenders do wonders for policy enforcement, unfortunately they may not
be a legal technique at your organization.