Look for mushrooms that are firm with a fresh, smooth appearance. Surfaces should be dry, but not dried out, and appear plump. A closed veil (underside of the cap) indicates a delicate flavor, while an open veil and exposed gills mean a richer flavor.
Refrigerate mushrooms for up to a week in original packaging until ready to use. Once opened, store in a porous paper bag; avoid airtight containers - they cause condensation, which quickens spoilage. Fresh mushrooms should never be frozen, but frozen sauteed mushrooms keep up to 1 month.