Keep the mussels refrigerated with a damp cloth covering the bowl. DO NOT store them in standing water. That would drown them as the little oxygen in the water will be used up by the mussels. Keeping mussels damp allows their gills to to absorb oxygen and breath out carbon dioxide into the air around them, thus keeping them alive.
Preparing them for eating
Rinse off the mussels under the faucet and pull off the byssus threads. If there are no threads to pull, throw the mussel away. Once the byssus threads are pulled, cook and eat the mussels as soon as possible for best flavor.
Simple way to cook mussels
Put them in a pot and just cover the bottom of the pot with water or wine or beer. Toss in a couple of cloves of garlic and some sprigs of any type of herbs, such as basil, thyme, oregano. Celery tops are also nice to drop in. Steam mussels until the shells open. The longer you cook them, the less tender they become, so don’t overdo. Serve with melted garlic butter or any other favorite sauce.