Potting soil should be stored in a cool dry place, such as a garage or shed, but not somewhere it can possibly freeze. If you store it outside, it may freeze during cold weather.  If you have plants currently using the potting soil, this could damage the roots of your plant. You can keep unused pots of potting soil inside a plastic bag that has holes punched into it so moisture drains through. Or, wrap each individual pot in newspaper and put all the newspapers together in another large paper sack. Make sure there’s enough space between the layers of papers so the soil doesn't get wet. Store this way until springtime.