A version of the Noguchi filing system has worked for me for years, and it's the only organisation systems that's lasted more than a year.
Take every document and store it in an A4-sized envelope. Mark the title of the project on the corner of the envelope, and store the envelopes in a row a bookshelf. Add any new document to the left end of the "envelope buffer." Don't attempt to classify documents. Whenever an envelope is used, return it to the left end of the bookshelf. The result of this system is that the most recent and frequently used documents move to the left, while documents that are rarely or never used migrate to the right.
This system works because it's designed with respect to our innate abilities and weaknesses; it's easy to roughly remember how long ago something happened, and easy to scan text. It's an easy system to keep in order. To clean your desk, tou grab all your files and stick them on the end.
Can you redesign your systems to make the most of your (or your customers') existing habits and strengths?