Managing project support materials is one area of the GTD system that I've never gotten comfortable with - in particular, the paper support materials. I follow the "plan B" method for managing them from the book - I separate them from my reference files and keep them close by. It works great when the project is active - it's when the project is closed, or worse, when it's a someday/maybe that I struggle.
When projects are completed, I typically mark the folder Inactive and move it to an archive file drawer. The problem with that approach is the number of files the accumulate over time. Eventually, retrieval becomes a problem.
If a document comes across my desk that I might want to hold for a future project, I create a new project folder then struggle with what drawer to put it in. In the past, I've:
1) activated a project, although I have no intention of working on it, just to keep the paperwork at hand to start the project up;
2) hidden it in the back of the drawer behind the project support documents; and
3) filed it with the other inactive projects.
I'm curious as to how others are handling this issue...