Same boat as Glenn and Clefmmann.
I save everything (including others' model throwouts) for repairs, scratchbuilding, conversion, or if someone else needs them.
Kit parts are stored in plastic storage boxes, divided by subject (cockpits, engines, cowls and intakes, etc), loose decals in ziplock bags according to nation, stored in metal biscuit tins (as are aftermarket sheets). Complete kit sheets bought off the net in photo albums (sticky page type).
Instructions are stored together in a folder, and box tops cut off like Glenn, aim being to collage them on a wall someday (...) though if more than one of the same kit made, only one is kept.