How do you store powdered herbs?

Cool, dark place. I think airtight glass jars would be good.

Actually, for long term storage, a freezer is a good place - the cold doesn't hurt them nearly as much as the heat and light will. If you buy herbs in bulk, take out what you will use for say, a week (keep in a glass jar, cool and dark place), then put the rest in a glass jar and put that in the refrigerator would be much better than where it may get too warm.

It's about not loosing the essential oils and such from the plants. Heat removes them, cool temps keep them intact.