Best way to store LPs?

If you store them flat they will warp.

If you store them, they must be clean and dry. Then they cannot be piled flat. Go perpendicular and in outer sleaves of plastic as inner of paper (acid free). So sayeth the Library of Congress conservator who say digital is not a reliable medium/mode of storage.

Al suggestions solid so far. Would go a step further by not only keping them standing on the bottom edge and keeping them out of high humidity but out of excessive temps and avoid all sunlight. If they have paper sleeves you might want to replace them with plastic sleeves and if the covers are imortant to you as well place them in plastic covers as well. In bulk they are easily affordable and worth the investment.