The same way you should store anything you don't want to break down, away from heat, light, oxygen, and water. So a sealed lightproof container in the freezer.
I read that the 25 series doesn't break down as easily as say, acid. Acid is a very fragile substance. 25i on the other hand, can take quite a beating. Can easily tank 100+ degrees Fahrenheit. Definitely wouldn't want lighting exposed for extended periods of time.
