Dishwasher. I use a plastic ball cap washer (purchased online). You can pre treat the hat before you put it in the dishwasher if you choose (for tough stains), and I use a non bleach, non citrus based dishwashing detergent. I run it on a normal cycle, warm but not hot water. I find a bowl that fits the inside of the hat relatively well and dry the hat in a place that has some air circulating, or I will just set it in front of a fan.