The first step in keeping mushrooms fresh for as long as possible is to make sure you're picking healthy-looking mushrooms.
Look for mushrooms that look damp, but not wet or slimy. If you're buying them in bulk or at the farmers market, touch the mushrooms to check that they are not wet; otherwise
The number one rule when it comes to keeping mushrooms fresh is to let them breathe. Storing them in an air-tight container or in a plastic bag will cause condensation and speed spoilage.
McGee writes that mushrooms, "should be loosely wrapped in moisture-absorbing packaging to avoid having the moisture they exhale wet their surfaces
Also, adding excess moisture, either by washing your mushrooms before you put them away or wrapping them in a damp paper towel, does your fungi no favors.
You can use the original packaging—the paper cartons with perforated plastic wrap—from the grocery store because they are generally designed to allow airflow.