Storage is of course a common issue for all types of 3D terrain, whether you're talking about cardstock, resin, plaster, or scratch-built (probably *least* so for resin - because the pieces are so damned expensive that few people can afford to buy enough for storage to become a problem
). The typical solution is big plastic bins to store stuff in. At least, the cardstock models will weigh the least, and are easiest and cheapest to rebuild if they do get damaged through careless storage. (Yes, they may also be slightly less turdy than the other options, but you'd be surprised at how strong a well-designed and well-built carstock model can be! Some people with experience in multiple technologies have reported they actually stand up *better* against rough player handling than either plaster or scratchbuilt models.)
A few creative storage solutions are possible as well. Buildings can often be stored inside of slighty larger buildings. Sometimes you get a "ruined" version of a building or city wall which fits neatly inside the undamaged version (allowing for a nicely dramatic reveal during the game). Some people have even started hanging models against a wall (especially if they're secured by velcro - one of the mounting options WWG offers). And of course, some are nice enough to find a place as a prominent display piece in your home, or even your favorite FLGS (the Maiden, for example, graces many a coffe table or book shelf...).
Do keep in mind that most of these models are very modular. So, unless you start gluing things down into huge dioramas, you really only need one or a few copies of each piece for most game purposes. Think "a few plastic bins with modular pieces", rather than "stacks and stack of dioramas" (although the latter has been known to happen as well
).