I try to pick up and scatter those little pellet "dryer" packs through my displays (discretely). I guess you can bulk order through Amazon but they come with clothes and stuff. I live in a humid area too. Also use a lot of fans anyway.
Tho I think to an extent the cracking, if it happens, won't be the fault of the collector, probably. I don't think I've read any thread here that ever said, "screw it, I'm gonna display my stuff in this nice southern window coz of the great light" or "my collection is right next to the jacuzzi!". I think it's a combination of aging and whatever HT uses in the first place. Too many threads on the 'net pointing out 20 year old toys, Barbies, and all, that are still in great condition after being thrown in toy boxes for years.
Srsly, how many of us have stripped figures and they're already stained from the cheap dyes HT uses? But, hafta say - LOL doing some kitbashes - I just haven't been happy with the 5$-$25 dollar bodies I've picked up. I always seem to drift back to genuine HT, so HT must be doing something right.