Look for firm, unblemished caps. The mushroom's surface should be free from moisture but not look dry. Avoid mushrooms with any sign of mold.
Always refrigerate mushrooms. Loose mushrooms keep well in paper bags in the refrigerator. Avoid airtight plastic bags because they will retain moisture and speed spoilage. Properly stored, fresh mushrooms will keep for five days or longer.
Don't clean mushrooms until you are ready to use them. To remove any bits of the peat moss in which they were grown, rinse quickly with cold running water or wipe the mushrooms with a damp cloth, paper towel or soft brush.
Using Dried Mushrooms
Uncooked fresh mushrooms don't freeze very well, but once they have been cooked, they can be frozen in small containers for a convenient, ready to use addition to soups, stews and sauces. Just clean, trim and slice the mushrooms; saute them in oil or butter and cool to room temperature. Then, package in airtight 1/2-cup or 1-cup containers and freeze. They may be kept several months and are best if thawed in the refrigerator before use.
Dried mushrooms are available in gourmet stores and in some supermarkets. You might find chanterelles, porcini, morels, shiitake, woodear, lobster, black trumpet or mushroom mixtures. They should be stored in a cool, dry place. They are best used in small quantities to add flavor and intensity to sauces, soups and stews. Rehydrate them in warm water until they are soft before adding to other foods. The soaking liquid can add flavor as well, but should be strained before using to remove any sand or grit.