I have always stored mine in a zip lock baggie in the freezer...I stock up this time of year when they have the school sales, and they all get tossed in the same bag! I'm sure I have some that are at least a couple of years old, but have never had a problem when I go to use them.
I am wondering if you have very low humidity in your home and maybe that's why they are drying out on you after opening them? Maybe you should store them in the freezer between uses, also, and see if that helps.