The biggest factor in increasing the storage lifetime of seeds is getting them very dry, and keeping them uniformly very dry.
After that factor, it's nice if they stay at a steady temperature, especially if that is a COOL temperature.
Dark storage is slightly better than light storage.
"Very dry" means 15 % RH to at most 30% RH. Desiccants are helpful if you want to double or quadruple seeds viable lifetime (but not all seeds can be kept for many years).
MAYBE some old seeds germinate better after an overnight soak in dilute hydrogen peroxide (0.1% H2O2)
http://garden.org/ideas/view/R...
Also, to avoid soil rot setting in before a "slow" old seed germinates, you can germinate seeds on a coffee filter in a plastic bag. Use hydrogen peroxide as above to further reduce fungus and bacteria and possibly "stimulate" the seed embryo. Put LOTs of stale seeds onto the paper towel, and inspect it daily. Pot up each seed as soon as the baby root (radicle) emerges. Even if only 1% of the seeds germinate, and they take twice as long as usual, if you start with 100 times as many "old" seeds as you needed, you can still get a crop.
"Seed life span approximately doubles for every 10% reduction in seed eRH. "
<... during storage. "seed eRH" = Equivalent Relative Humidity>
"Once transferred to the seed bank, collections can then be dried to
around 15% eRH
(4-7% moisture content depending on seed oil content),
the recommended moisture level for long-term conservation of orthodox seeds"
http://www.kew.org/sites/defau...
http://garden.org/ideas/view/R...
http://garden.org/ideas/view/R...
http://garden.org/ideas/view/R...
