The climate control should be well maintained and dust free. Each piano should be stored individually so other items are not places on top and it should be protected inside and outside. You need to make sure it is away from any water sources.
When storing a piano you should consider that moisture can still enter a storage unit. You should check for drainage and also make sure it is elevated off the floor.
Have the piano on a pallet or otherwise lifted off the floor so that the feet and legs cannot be damaged by spilled water or condensation. Have the keys covered to protect them.
