Loved caps carry more than good memories. They’re seldom fresh, always sweat stained with sometimes a touch of mustard on the brim. Really Simple offers a quick way to freshen: Add a drop of mild laundry detergent to a sink or basin filled with cool water. Soak the hat for ten minutes, rinse thoroughly and blot dry with a clean towel. Avoid twisting the brim and hang to dry. See the full article for deep clean tricks, cautions and tips using a dishwasher for baseball caps!
From:: Washing a baseball cap.