From Cooking School Secrets for Real World Cooks, by Linda Carucchi

To prevent mushrooms from absorbing oil when you sauté them, be sure to heat the pan, then add the oil, and then wait until the oil is hot before adding the mushrooms. Once you add the mushrooms, let them sit, undisturbed, so they can brown. Don't start stirring until they brown on the bottom, unless you want them to steam instead of caramelize. Follow these directions and you'll end up with flavorful mushrooms and nice fond— or caramelization—in the bottom of the pan.