Sterilize any wide-mouth jars and lids in boiling water (old mayonnaise jars or jelly jars are fine). When cool, press in butter or margarine to within one inch of top while squeezing out air bubbles. Fill to the very top with cool, strong brine (add 1/4 cup pickling, kosher, or uniodized salt to 2 cups boiling fresh water. Let cool completely before adding to butter). Butter packed in brine will keep up to 6 months stored in a cool place. Dip out butter as needed with a clean knife. Top up brine solution with clean, fresh water.