I might add to what has already been mentioned.....
....From the writings of Bob Hayden
".......Old decals may fall apart when you soak them in water. Coat the decal film before application with a clear gloss, or with Microscale Superfilm.
Decals that refuse to stick can be applied with a diluted solution of white glue. Yellowed decals can often be saved by taping them to a South-facing window for a day or two, allowing the sun to bleach them.
.............Temperature and humidity fluctuations are the enemies of decal film. Store decals in a cool, dark, dry place, in an air-tight container.