cool dry place in a airtight container is good for this and most other solid chemicals;
by cool i always take it to mean less than 16 degrees centigrade [fridge is good; cool cellar etc] and by having an airtight container you stop degradation due to damp, oxidation etc [though not too much of a problem for this one]
this applies to many medical drugs as well BTW
[for serious flash value add a little packet of silica gel out of a vitamin tablet container to it which should keep it dry too:wink:]
