Brush off any peat moss with your fingers or a damp paper towel, or rinse the mushrooms briefly under running water and pat dry with a paper towel. Do not soak mushrooms in water as they easily absorb moisture. If the stem is tough, trim it before using. For shiitakes, stems should be removed before use. For portabellas, gills may be removed upon preference, as they store a large amount of moisture. Many people prefer to keep the gills intact for more flavor. 

Courtesy of the Mushroom Council