Documenting architecture

One of my side projects at work right now is documenting the architecture of a product that has already been built but will be going through a re-architecting with a focus on a more robust schema and applying some of the learning we've gone through in discovering exactly how our product is being used and ways in which our users want to extend the platform. SaaS and SOA are two good buzz words we'll be throwing around a lot, although to be honest we've been in the SaaS model for years now, just not following all of the best practises. (examples, check out litwareHR)

So despite documentation being at the heart of the architect's role I find it extremely difficult to find good documentation on how to approach a task like this. I have Craig Larman's book Applying UML Patterns which I've enjoyed, but I still find myself grappling for where to even begin sometimes.

These articles on IBM have been good reads for this and I'd reccommend giving them a read if you are facing similar challenges.

Part1
http://www.ibm.com/developerworks/library/ar-archdoc1/index.html?S_TACT=105AGX20&S_CMP=EDU

Part2
http://www.ibm.com/developerworks/library/ar-archdoc2/index.html?S_TACT=105AGX20&S_CMP=EDU

Part 3
http://www.ibm.com/developerworks/library/ar-archdoc3/index.html?S_TACT=105AGX20&S_CMP=EDU

I'm assuming there will be more of these which I'm looking forward to.