Process improvement alone can be a challenge.  For those trying to improve or at least understand what is possible at their organization, we highly recommend the CMMI site at Carnegie Mellon's Software Engineering Institute (SEI).

Microsoft is sharing best practice information it has gathered from its own architects.  The Architecture Journal is not security specific, but a good place to start.

The following papers or books are targeted to Architects faced with security challenges and go well beyond the standard security vendor marketing statements and fluff.

High-Assurance Design: Berg, ISBN 0-321-37577-7

Threat Modeling: Swiderski, ISBN 0-7356-1991-3

Writing Secure Code: Howard, ISBN 0-7356-1722-8

Improving Web Application Security (Threats and Countermeasures): Microsoft, ISBN 0-7356-1842-9

Addressing Application Security Challenges: Graf, Watchfire, June 2005