Mushrooms do double duty as both a seasoning and a vegetable in their own right. In fact, white button mushrooms contain umami, a natural flavor enhancer that boosts the flavor of any food that the mushrooms are cooked with. When choosing fresh mushrooms, look for those that are firm and slightly moist, with no signs of decay. They should be heavy for their size and smell like the woods. TO ensure freshness, check the gills on the underside of the mushroom. If they're tightly closed, the mushroom is young, mild-tasting, and will last longer; if they are open, the mushroom is more mature and will have a more concentrated flavor but will not last long once you get it home.
Use older mushrooms soon after purchase. If you can, choose so-called "wild" mushrooms, most of which are now cultivated. Though they're more expensive than other varieties, shiitakes, creminis, chanterelles, and other wild mushrooms have more intense, interesting flavors, and a little bit tends to go a long way.
For wild mushrooms, please note that only mushroom experts should pick or use fresh mushrooms from the wild. Identifying mushrooms in the wild can be very tricky, and some varieties are poisonous.