Do not prepare mushrooms until just before using. When cleaning mushrooms, avoid using water because it will reduce the flavor. Mushrooms are easily saturated with water and tend to become mushy if soaked. Use a soft brush to clean sand, grit and other debris from the pits and ridges. If you must use water, use as little as possible and dry quickly with a paper towel.
Cultivate mushrooms will require less cleaning than wild mushrooms. Because they are grown in a controlled environment in sawdust, dirt, and other coarse mediums they do not tend to get as dirty.
Wild mushrooms require a little more cleaning because of the environment they are exposed to. Start by cleaning as stated above. Use a damp paper towel to wipe off anything that could not be brushed off. In All About Mushrooms
, you will find more details on cleaning wild mushrooms thoroughly.