Julia Child is very helpful on this subject.
"Like all of the recipes in Mastering the Art of French Cooking, the secret is in the details. ?The mushrooms must be dry, the butter very hot, and the mushrooms must not be crowded in the pan,? the book advises. ?If you sauté too many at once they steam rather than fry; their juices escape and they do not brown.?
Julia?s master recipe uses a 2 to 1 ratio of butter and oil in the pan, but just olive oil works very well too. Once the butter has stopped foaming or the oil is very, very hot, the mushrooms?washed, thoroughly dried and sliced?go in and patience becomes the name of the game. For the first few minutes, they will absorb the fat until the skillet is dry. At this point, the mushrooms emit a funny little squeaky sound as they are shaken and tossed around the pan. There?s an instinct here to add more butter or oil or lower the heat to medium?resist it and wait for the magic.
In another minute of two, drops of fat will start to appear on the surface of the mushrooms and they?ll begin to brown. Keep a close eye on them and remove them from the heat once they?ve attained a nice color and start to release their juices. Just a little salt and pepper is all that?s needed for a perfect accompaniment to grilled main courses, but a sprinkling of herbs like thyme or chives is very nice too."
She is most precise about these things.