Add warm water to a bucket. Wet a sponge in the bucket then wring out the excess moisture.
Place three or four drops of shampoo on the moistened sponge. Gently scrub stains on the top and underside of the bill. Rinse the sponge as stain material lifts away. Apply more shampoo and continue to spot-clean the bill of the cap.
Rinse the shampoo off the bill by wiping it with a damp cloth.
Secure the baseball cap on the upper rack of the dishwasher with two or three wooden clothespins. Wash the cap using dishwasher detergent and white vinegar added to the rinsing agent reservoir to help remove sweat stains, brighten the colors and get rid of sweaty odor.
Take the cap out of the dishwasher as soon as the rinse cycle completes. Shape the cap with your hands and place it over a clean coffee can to dry.