All plastics will degrade with time. The worst 3 items to accelerate the degeneration process are:
[1] Sunlight (Ultraviolet is a killer) or other bright light with an ultraviolet component.
[2] Heat
[3] Humidity & Dirt/Contaminates
Be carefull cleaning nitrate dice as the crystalized plastic is slightly soluble in water. The degradation byproducts of one die can cause degeneration in perfectly good dice if it gets on them. The byproducts of nitrate plastic degeneration is especialy bad for almost evrything else so should be stored seperately (they do not play well with others).
The new CelluloseAcetate dice will degrade with Ultraviolet light and heat. They do not crystalize but will inmplode, becoming warped with sunken in faces and will darken in color. They will give off a vinegar smell as acetic acid comes off of them. Acetate dice that have started to get "vinegar syndrome" should be stored seperately from otherwise good acetate dice as they will caus the good dice to go bad as well.
A bag of zeolite resin in with the dice might be helpfull to absorb humidity and degradation byproducts.
Hope this helps