We keep our decal stock in sealed ziplock bags in a room with as constant a temperature as can be managed. A decal sheet comprises three main constituents: the backing paper, the glue and the ink. Decal paper is by its nature extremely absorbent - notice how quickly it "wicks up" water - therefore if you keep it in damp conditions it will absorb moisture and expand, contracting as it dries out. The glue is a thin coat of gum arabic, applied evenly (if the decal paper is of good quality) to the paper. The inks are acrylic and printed on top of the glue layer. The acrylic inks don't absorb moisture and therefore if the paper is allowed to expand and contract due to humidity (and to a lesser extent, temperature) fluctuations, then the ink will tend to crack and craze, breaking the integrity of the printed image, so that the decal breaks up along these fracture lines as soon as it is dipped into water and the glue layer dissolves.
Our printers have said that they have to keep an eye out for weather changes during printing, as on a damp day the paper expands considerably and if this occurs during a multi-layer print run (each colour is a separate printing operation and each layer must dry out before the next is applied) it can lead to miss-registration of images, particularly with thin lines etc. In extremely humid conditions, the gum arabic partially dissolves in the moisture absorbed by the paper and seeps into the paper structure(normally it sits on the surface). When the paper is wetted to release the decal, the glue which is inside the paper refuses to let the decal go, rendering the decal useless unless it is soaked for a prolonged period - unfortunately long soaking dissolves and disperses the glue, so even when you get the decal separated it won't stick to the model!
So, stable conditions giving minimum moisture fluctuation are the best. A sealed ziplock keeps moisture out (or locks in whatever damp is in the atmosphere when it is sealed), thus the paper doesn't expand and contract and the glue also stays where the paper manufacturer intended.
Decals included in kits should also be kept in a sealed bag – if the kit doesn’t include one, put the decals in a ziplock bag and back in the kit box. If you buy a second hand kit (or even one that has been in the shop for a while) you will often notice that the decal sheet is curled – in that case you may well have a problem when you come to use them as the curling is caused by a damp sheet drying out……