Mike -
I have a PST folder with my name on it.
Inside that I have folders for each client.
Inside those folders, I have a "General Reference" folder; plus a "Process & Procedures" folder; and a folder for the Daily/Weekly Status Reports that client generates.
Also inside the Client folder, I have Project Folders that are named with the Job Number our Agency assigns to each Project as it opens up.
Inside those project folders - I collect all the e-mails from the start of the project to the final billing.
When the project is complete - I print the contents as a PDF and save it to our Department Server, under the Client's file there. This way, anyone else in the dept can review the history of the project, after I've (possibly) moved on to another client.
I have a parallel hierarchy set up for departments that I manage, and people reporting into me.