Inspect the pores, they shouldn't be too open, rather they should be tight and compressed. Scratching them with a fingernail they should come off easily. If the pores are tough, consider drying the mushrooms or saving them to make stock.
Slice the mushrooms as thin as possible, I like to do this using a mandoline slicer. If you don't have a mandoline, just slice them as thin as you can.
Cook the mushrooms however you like. Adding a splash of wine to the pan can help them from drying out.