The button mushroom is the iceberg lettuce of the fungus world. Unlike morels and porcini, the button mushroom doesn't have a distinctive flavour or texture, and because it's always in supermarkets, people take it for granted, using it only when "exotic" mushrooms are not available.
It takes skill (and other ingredients) to bring out the best in a button mushroom. Like other varieties of edible fungus, the button mushroom contains a lot of water, which it releases when cooked. There are ways of dealing with this. The most logical method is to not cook them; instead, soak the mushrooms in a flavourful vinaigrette made of extra-virgin olive oil, fresh lemon juice, grated lemon zest, chopped parsley, minced garlic and salt and pepper. Refrigerate for a day or two before serving with bread to mop up the vinaigrette.
Another way of preventing the mushrooms from releasing their flavour liquid is to cook them "dry". Brush the whole mushrooms with olive oil, sprinkle them with salt then grill them over hot coals - the intense, dry heat will concentrate the flavours. Or, slice the mushrooms then sear them very quickly on both sides in a lightly oiled pan. Don't crowd the pan, or the mushrooms will steam in their own vapour, rather than sear.
You can also cook the mushrooms long enough, so they re-absorb the liquid. My favourite way of doing this is to cook them with butter and cream. Heat a skillet, add a good chunk of butter, along with chopped garlic and shallots. When the aromatics are slightly softened, add the mushrooms (either whole small ones or sliced larger ones). Season with salt and pepper then let them cook over high heat, stirring often. The mushrooms will release a lot of liquid, but be patient - let them simmer until the liquid is gone. Add some cream to the skillet and continue to simmer until it's absorbed. Sprinkle with chopped parsley then serve.
When buying button mushrooms, look for caps that close tightly around the stem; if the caps are open, the mushrooms are old. The mushrooms should be pale off-white. If they seem dirty, plunge them briefly into a bowl of cold water, then remove them and dry them with a clean dish towel.