Heat and light accelerate the deterioration. They can be somwhat stabilized by coating with a clear laquer or nail polish. Thet still may deteiorate on the inside but will at least hold together with the protective shell. There is not a lot of material avaiable on nitrate plastics. A good link with info on nitrate dice storage is:
http://dice.onaraonline.org/care.php
the page is a work in progress but curently has info on nitrate with other plastics info to com. I have looked up cellulose nitrate conservation and most of the available info is way outdated and of no use. The best bet now is the clearcoat/shell to preserve the dice and keep them in a cool. dry, dark place. Most collectors with old nitrate dice do not want them clear coated as it decreases the value because they are altered, but I would rather have coated dice that little piles of dust. Also the dice should be cleaned with a damp swab and then with a dry swab to wipe the acid off the surface. The decomposition products will damage other dice and accelerate thier decomposition. A packet of zeoliteresin in with the dice will help absorb some of the decomposition products from the dice. The more modern Cellulose Acetate dice will also decompose if exposed to heat and sunlight giving off a vinegar smell.
hope this helps