How can I preserve opened Lego packages?
Well I used to keep mine as well. One thing I found very good was to take a razor or a box cutter and cut the front and backs off! this may seem obvious or defeat the purpose, but it made them easier to store. I would stack them all and slip them on a shelf or under my bed to keep them flat. This proved to be a very effective solution. Also, you could try to collapse them at the ends to have them take up less room as well.