Keep the Original Store Packaging
You can't see it, but the wrap on packaged mushrooms actually has tiny holes that prevent damaging condensation and gasses from building up. You can, and should, store commercially grown mushrooms (presliced or whole) in their original container and wrap in the refrigerator; they'll keep up to one week.
Keep Moisture at Bay: Wrap Loose Mushrooms in Dry Packaging
Storing mushrooms in a damp towel will shorten the length of time they stay fresh. Instead, refrigerate loose mushrooms in a brown paper bag; it will absorb any moisture and allow them to breathe. A partially opened zip-top plastic bag will also work.
Avoid the Crisper
The refrigerator's vegetable bin is a moist place—not a friendly area for mushrooms. Instead, keep them in a spot where air can circulate, like an open shelf.
Keep Mushrooms Away from Pungent Foods
Mushrooms can take on the flavors of strong foods that are stored nearby. Keep them away from onions, garlic, and other foods with strong odors.
Rinse With Water
Clean mushrooms under cool, running water right before you're ready to use them; then pat dry. It's a myth that mushrooms easily absorb water, but if they're rinsed too far in advance, they will discolor and deteriorate from the moisture. Cultivated mushrooms grow in sterilized compost, so you can also clean them by simply brushing any bits of "dirt" with a clean, damp towel.