Most clams available commercially have been cleaned, but if you dig your own, you must remove the sand and grit from inside the shell. Either use salt water or make enough saltwater brine (1 cup of salt per 3 quarts of water) to cover clams. Soak clams in salt water in the refrigerator a few hours or overnight. This is particularly important for clams with protruding siphons. For tightly closed hard-shell clams, it may be enough to simply scrub the shells to remove grit. After cleaning, refrigerate clams in a bowl, covered with a damp towel. Clams will keep for up to a week, but should be cooked as soon as possible. To freeze, remove clam meat from shells and wrap it in freezer paper or plastic; then over-wrap with a plastic bag. Store for up to two months. To thaw, place frozen clams in the refrigerator overnight. To thaw faster, wrap clams in waterproof plastic and place in a sink with cool running water, allowing about 30 minutes per pound (450 grams). For fastest thawing, use the defrost cycle of your microwave, letting clams rest between brief zaps (as one minute defrost to one minute resting).