Rice grains are a good start because they absorb moisture.
So do that, then put the whole thing in an airtight container or a film canister, and in the fridge like crispi says.
5 years from date of seed harvest is full germination. Then after that, you can start to see germ rates decrease, like at a rate of about 10% per year, but good storage practices will keep your %germ pretty high for quite a while.