It is best to purchase clams just before cooking them so there should be little storage to worry about. Clams are purchased alive and must be kept alive. If you do purchase them several hours in advance, they should be kept cool and moist on ice or in the refrigerator. Don’t wrap them in anything airtight; they need oxygen to survive. Just before cooking, scrub them with a stiff brush under cool running water and trim off beards if they have them. If any are open, tap the shell. If they don’t close tightly, discard them. If any don’t open when cooked, discard them. All of the above goes for mussels as well.