The key question in filing or storing material is not, “Where do I put it?” but rather, “Where do I find it?”
You can have the most sophisticated storage systems available, but if you don’t know where to find what’s inside, you’re no better off than having stacks of stuff all over the place.
One of the most useful ideas within GTD is the simplification of the personal filing system. How do you file your own reference materials?

Two parameters drive the system:
- It must be easy to file materials otherwise you won’t
- It must be easy to retrieve materials, or you won’t trust the system.
Having used all sorts of elaborate, cross-referenced, index-card, electronic, with filing cabinets, bankers-boxes and card-box systems, I can tell you, they don’t work. There is nothing simpler than alphabetical order.
This is the beauty of the alphabet: categorize what you have in your hand, put it in a file folder, label it and file it under the first letter of the label. All in order and quick to retrieve. When you need something, you will find it in one of a couple of places.
For example, my gas bill will be under G for Gas or D for Direct Energy, my supplier. A more complicated system might have the gas bill filed under Bills>>Home>>Utilities>>Gas. More difficult to recall, and difficult to set-up. You have to know the rules and categories ahead of time and have some way of keeping track of them.
Electronic file systems sort alphabetically by default, so applying the system to your electronic documents should not require too much brain power.
You can download a free pdf from Davidco, with David Allen’s thoughts and ideas on General Reference Filing.