First of all, be absolutely sure that your fish purveyor is reputable and takes very good care of his stock. Buy only clams that are tightly closed. Mussels may be slightly open but should close up tightly when tapped. Anything else means the shellfish is not alive and should not be purchased. Plan your buying trip so it’s the last stop and you’ll be arriving home soon after. Rinse the shellfish with very cold water and drain well. Arrange in a shallow pan, cover lightly with a dampened cloth or paper towel, and store in the coldest part of the refrigerator. Don’t refrigerate in a plastic bag. Use the shellfish as soon as possible and definitely within 48 hours.