I used to save the boxtop art - didn't bother with the ones that only had a picture of the assembled kit. Then I'd keep one box end and, if one of the sides had pics of other kits, I would save that as well (so I'd know what else I wanted to buy). Then I would paste the box end onto looseleaf paper and keep them in a 3-ring binder. I haven't built for over a decade, so I don't kknow where that binder is right now. But I do have all the instruction sheets from all the kits I;ve ever built. Well, most of them. And that's a huge stack of sheets. I also had a box with leftover decals - it wasn't airtight, so most of them are no good any more. But I would write the kit description on the back of the decal sheet in pencil whenever that info wasn't already there. I have a couple of nut & bold wall cabinets that I used to sort spare parts, either by type or by kit.
Sadly, a lot of the boxart I kept was ruined when the exhaust pipe from our furnace came loose and spewed soot all over everything in the basement. Even now, almost 10 years later, if I take a kit off of the shelf I have to wipe it clean, or else the coating of soot will get over everything. I still have a bunch of the box tops...they're just substantially darker than they used to be.