The kids take digital pictures of their creations, they sit on the shelf a brief time before they are pulled apart and turned into something new.
I keep all the instructions in a 3 ring binder, slipped into page protectors so when they want to rebuild the item that the set came designed as they can. Or they make their own creations.
They just got great chest of drawers made by Lego that stores their Lego pieces and has a couple plates to leave things set up on if they want to keep them built.
Growing up, all of my Legos were put in a large Tupperware tub that I dumped out on the floor when I wanted to build something. I had a big piece of some thin wood, like paneling or something I think, that I would dump them on and could build them so they stood up since we had carpet. I kept all my instructions in a big manila envelope at first and then later put them all in a binder with pages protectors. My kids still play with all my old Legos when they go to my parents house :)
Sticking to nothing but the pre-designed build is always so borning IMO LOL