How should baking soda or baking powder be stored?

Store baking powder and baking soda in air-tight containers and place them in a cool, dry place.
 
For optimum shelf life, transfer baking soda from its cardboard container to an air- and moisture-proof container. Keep in mind that baking soda is a potent odor absorber. If you don't want your baked goods tasting like the odors that surround it, keep a lid on your baking soda. Moisture will also compromise baking soda.   The same goes for baking powder.
 
Source(s):
Survivalcenter.com "Food Storage FAQ: Cooking Staples"
Monkeydish.com "Baking Powder and Baking Soda"