It is difficult because there are so many components, so many services to integrate with, and so many potential knobs to turn. This is not a web app and a database. This is a biggin. Getting the partitioning in the right places is crucial.
When I am in the details I panic that I can't see the forest from the trees.
When I am at 20,000 feet, I panic because modeling and hand-waving doesn't cut it.
The only solution of course is to alternate between zooming in and out like this until you are done.
I got a third of the way through my Masters of Software Engineering at OMSE before I "paused" to join an OSS ESB startup (now defunct). I learned a couple of things there - one topic that I retained and applied is Views.
I went back to this today because I had the "too much detail panic". I came across and ordered Software Systems Architecture which looks quite good. Take a gander at the Quick Reference Card (requires you to give up an email address).
I like the looks of it because it builds on the "views" concept and gives some more detail (i.e., perspectives). Anyway, unless someone comes up with a better idea, I will just create the hierarchy with the viewpoints and perspectives in our wiki and then I think I will feel better zooming in and out as I will have some structure and cross reference. Perhaps it will start to become paint by numbers at some point.
Let me know if you have other ideas.