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Some sleeping bags come with a larger sack which does not unnecessarily compress the sleeping bag. So you might get a linen bag of around 50l volume (depends on the size and type of the sleeping bag). Furthermore, it is best to store it in a dry place, especially if it's a sleeping bag with down stuffing.
When you put the bag into a compressing sack, the insulation fibers take some damage. (Synthetic fibers are less resistant in this regard than down.) The longer and harder you compress the insulation, the more damage; the insulation slowly loses loft and gets "colder". Therefore it's best to keep the sleeping bag hanging or lying freely, possibly in a bigger sack. That's about the only thing that matters, plus maybe the decent humidity mentioned by Thomas.