Three-hole punch, pocket folders with brads in the spine, and three-ring binders. Each folder gets labeled with its subject, and everything relevant to the subject gets hole-punched and added to the folder. When the content outgrows the folder it gets moved into a binder that gets labeled on the spine. Documents sent digitally get printed and treated the same way. For sheets that can’t be hole-punched like a birth certificate or a car title, I use either the pockets in the folder or plastic page protectors that can go into the binder. For sections pertaining to specific events, like initial purchase and financing documents and maintenance visits for a car, I use tape tabs to mark the first page of whatever came out of that event. (You could use dividers made for three-ring binders, too.) It’s a good system; everything you need for it you can get just about anywhere, it doesn’t take much maintenance, and lookups are pretty close to trivial. Too, unlike a filing cabinet, it’s portable; if you need to take a set of documents somewhere (reference, tax preparation, etc), they are durably collated in a form that will easily withstand travel.