In storing yeast, moisture is your enemy. If you keep the dried yeast frozen or refirgerated in a sealed and air-tight container, as Jill described, you should have no problems. The freezer is generally a drier environment with a more constant temperature than a refrigerator where the door opens and closes, the temp flucuates and the humidity (again, your enemy) rises and falls with the moisture evaporation of fruits, veggies, juices, soups and the like.
Jill, I have not tried Fermipan. Where do you buy it? I get the SAF from Whole Foods in NJ but they don't always have it.