Heating polyester causes it to shrink, so this is an effective method for bringing a larger hat down to a smaller size. Take care not to place polyester hats into the washer or the dryer, since the aggressive action of these machines can damage the hat.
Polyester melts at 446 degrees Fahrenheit, so keep all temperatures below this level.
Step 1: Preheat the oven
Preheat the oven to 300 degrees Fahrenheit.
Step 2: Wash the hat
Run hot water in the sink, and add a tiny amount of laundry detergent. Wash the hat, and then rinse well with hot water. Only wash the hat by hand, since using a washing machine can cause damage to the hat.
Step 3: Put the hat in the oven
Place the hat into the oven immediately after washing it, without allowing it to dry. Leave the hat in the oven until it is dry.
Step 4: Try the hat on
Remove the hat from the oven, and allow it to cool. Try the hat on and see if it fits properly.
Step 5: Repeat as needed
If the hat is still too big, rinse it again in hot water, and place it back in the oven. Continue wetting and heating the hat until it is the proper size.