Look for mushrooms with a fresh, smooth appearance, free from major blemishes, with a dry (not dried) surface. A closed veil (the thin membrane under the cap) indicates a delicate flavor; an open veil means a richer flavor.
Keep mushrooms refrigerated. They're best when used within several days after purchase. Do not rinse mushrooms until ready to use. To prolong shelf life, store fresh mushrooms in a porous paper bag. Always remove plastic overlay from packaged mushrooms. Avoid air tight containers -- this causes moisture condensation which speeds spoilage.
Gently wipe mushrooms with a damp cloth or soft brush to remove occasional peat moss particles. Or, rinse with cold water and pat dry with paper towels.
Fresh mushrooms don't freeze well. But if it's really necessary to freeze them, first saut? in butter or oil or in a non-stick skillet without fat; cool slightly, then freeze in an air tight container up to one month.