Mushrooms bruise like ripe fruit. Handle each one once. Harvest into shallow trays, never deep sacks, and keep trays out of the sun.
Grade on a clean table by size and by whether the cap is closed or open. Trim stems level. DMR's oyster folder says spread the mushrooms on a cloth for about two hours so surface moisture evaporates before packing. Never wash mushrooms: a water film on the cap is where bacterial blotch spots start.