A museum curator would want them stored on glass shelves with climate controll. For the mainstream collector display trays made from acid free mat board padded with cotton would be a good choice. If you notice the cotton or mat board starting to turn yellow (will hapen around decomposing/crystalizing dice) replace it as the acid byproducts that are causing the yellow are bad for the other dice. Polyurathane foam is ok short term baut is bad long term as it breaks down and eats the dice. Polyethalene or "ethafoam" is supposed to be archive safe but no long term studies are available on it.