I have struggled with this in regard to paper resources, tools, craft supplies, cloths, memorabilia, etc.  The paradigm that I have come to use is to be outcome oriented.  I ask myself what outcomes of keeping the item would please me and whether I am  committed to pursuing them in forseeable time frame, can only decide after or on a certain date, or just want to keep the idea alive or need time to develop the idea.   Then I ask myself if you  have the necessary resources, resources can be tangible (time, tools, money, a helper) or intangible (like attitude, frame of mind). Then I file the item in a manner in which I can find it and link it to the project.   For the sake of demonstration I will call it "Nativity Scene 2001 diagram ".  Here are some ideas and how I would implement the storage:
Desired Outcome:  I have unique Nativity display by DEC 1 in my yard every  year  I set up a huge  xmas yard display and add on to the previous year's scene with a new feature. I am committed to this.  I have the time, talents, etc.   I know that I need to look at the diagram in September to make a plan. I put a note in September calendar  or my tickler and file the item under Christmas or Nativity Scene in my general file. I might also put a note in January or on errands list to look for items in the January sales for the next season.
OR:
Desired Outcome: I want to be able to find the diagram if I ever decide to make one again because I am a hobbiest wood worker and keep various diagrams and patterns for things I have made or want to make.
I have  many similar, patterns and diagrmas, so they are in their own box or drawer labelled Woodworking: Patterns and Diagrams.
OR:
	 Desired Outcome: I want to be recognized for my effort and remind myself of how successfully I  carryout my work.  The Nativity diagram was for a stage set for one of my productions.  I put the title on a list title Completed Projects 2005 in @LISTS, with the title  of the project, the date , the highlights,  etc. Then file the paper copy  under the title in A to Z. I also create and define Project called "Year End Report 2005" the next action for it  @computer:tidy Year End Report-see Completed Projects 2005 in @LISTS .
OR:
Desired Outcome: I want to remember the resources I have if I ever decide I want to set up a Nativity Scene. Chances are low I will do the project, but if I want to do it I want to be certain I can the design for it.  I would probably only do this if I was doing something for Xmas, so I file it under Xmas and just in case I forget I will put in my filing system a "dummy" folder that says on the file head "Nativity Scene Diagram-see Xmas."