We buy LTOC (low total organic carbon) grade, which is better than nuclear grade. It comes in 5 or 7 cubic foot fiber drums, with a plastic bag liner sealed with a cable tie. We've stored it for as long as two years without degradation.
If not an option, re-sealable bag, try to iron it shut. Vacuum seal bags are best bet.
Try not to buy more than you can use in 6 months, many vendors package it in 20 oz portions which is exatly what fills a refillable 10" vertical cartridge tightly.
If you must buy it in larger quantities I recommend repacaging it in 20 oz bags vacuum sealed with a Sael A Meal type device then store it in the back of the refrigerator so it stays damp, cool and out of direct lighting. Dryness, heat and lighting will kill any DI resin. Most resins have a proven 6-12 month shelf life even when properly stored and degrade with time. Often short DI life is attributed to old age or improper storage or both.
I make about 100 gallons a month and my DI cartridges last close to a year so I don't even buy a replacement until about the 9 month mark.
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