Between hunting seasons and shooting sessions seal your unused slugs in a zip lock bag and pack them in a styrofoam cooler.  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.