What is the proper way to keep a hog casing for long life span?

I keep them in the fridge, in a sturdy zip bag completely covered with kosher salt. They'll last a year or more that way. You can keep them in a salt brine in the fridge, too, but the salt-packed method takes less space. Soak in a couple changes of water before use. Freezing isn't recommended.

http://www.sausagesource.com/natural-casing-faqs#Storage