Between hunting seasons and shooting sessions seal your unused slugs in a zip lock bag and pack them in a sealed ammo box. Store the cooler in an area of your home where the humidity is as low as possible and the temperature stays between 50 and 70 degrees Fahrenheit year-round. The more stable you keep the temperature of your slugs, the better they will shoot. When stored properly, slugs will shoot reliably even after 15 years.
After the season any ammo that was exposed to above normal dampness— Remove the ammo from the boxes. Let the ammo and the box exposed to the warm dry air in your home during the months of Jan & Feb. The low humidity present in most homes over those two months will remove all the moisture that was absorbed, by the powder, while hunting. Remember slugs are open ended, they are not sealed like the rounds used to hunt waterfowl, turkeys, etc.
Place the ammo back into the box, place the box in a zip-lock bag (the smaller the bag the better). Now placed the sealed bags in a seal able type ammo storage box, Styrofoam cooler, and keep the box where it is cool and dry.
Unless you open the plastic bag, the ammo will last 10 to 15 years. Next year if you use them, do not open the bag unless you are going to fire them. The extra rounds you carry hunting can just as easy stay in the bag, in your pocket as not. This drying out procedure can be done more that once