Store it in any airtight container (ziploc bag) and it should be fine for quite some time. You will want to make sure it is completely airtight though or it will start to clump together.
Even if it does clump, it will still dissolve in boiling water. I like to divide it into separate baggies for starters, so all I have to do is put one bag into 2 quarts of water, boil, chill and pitch.
