Buy mushrooms that are firm, even colored and have tightly closed caps.  If gills underneath are visible, the mushroom is pasts its prime.  For cooking purposes, buy those of similar size so they will cook evenly .  Store in a single layer on a tray.  cover with a damp paper towel and refrigerate for up to 3 days.

Never soak mushrooms, they will absorb water and turn mushy.  Either "sweet" with a soft brush or take a damp paper towels and remove dirt.  Trim 1/4 inch off the stem.  When sauteing, be sure the pan and oil are hot.  Mushroom give off a lot of liquid; stir and cook until liquid evaporates.  They are a good appetizer when dressing with vinaigrette and refrigerated for 2 days.