I usually divide up any big batch of seed, into 2"x3" ZIPlocs, with printed labels, so they are ready to trade or give away.
Usually I store those in tightly sealed plastic tubs
with some desiccant in a paper packet.
Cool and DRY are the biggest factors. From what I read, they don;t varying temperatures or humidity either. After cool and DRY, darkness is supposed to help.
If you can keep them
dry in the freezer, no condensation but not dessicated by the defroster either, they might last even longer than when kept cool.
Here's a few links and quotes to the Kew Royal Botanic Gardens' Millennium Seed Bank information about collecting and storing seeds.
Kew Royal Botanic Gardens Technical Information Sheets about collecting and storing seeds, written by Millennium Seed Bank staff.
http://www.kew.org/science-con...
post-harvest handling for seed collection:
http://www.kew.org/sites/defau...
"Seed life span approximately doubles for every 10% reduction in seed eRH."
("eRH" is equilibrium Relative Humidity.)
" Once transferred to the seed bank, collections can then be dried to around 15% eRH (4-7% mc depending on seed oil content), the recommended moisture level for long-term conservation of orthodox seeds.
("mc" is moisture content , % water by weight.)
collecting seed in the field:
http://www.kew.org/sites/defau...
