Many of us store our flour in a sealed container to keep out the weevils. Similarly, to maximise the longevity of unused potting mix and keep out unwanted pests breeding in the mix, reseal bag and store in a sealed container. Place container in a dry, sheltered area away from the sun and moisture, preferably indoors.
Remember potting mix does degrade over time. If you have had a bag in the shed for over 12 months, spread the old mix onto a garden bed and buy a fresh new bag of potting mix for potting new plants. The old adage applies, ‘fresh is best’.