To prep button mushrooms, simply trim the stems. They’re usually clean enough without being washed.
Wash gritty mushrooms just before using them. Swish them in a large bowl of cold water, then lift them out. Repeat with fresh water until the mushrooms leave no sand residue in the bowl.
Black trumpet mushrooms bleed black when cooked, so blanch them in boiling water before sautéing them with other mushrooms.
To get porcini to soak up less oil and butter, add a pinch of salt when cooking them. Cover the mushrooms until they start to exude their liquid, then uncover them and let the liquid evaporate.