With the right care, a leather hat can retain its great looks for many years. Taking the time to remove sweat stains from the hat on a regular basis is one of the best ways to keep it looking like new. Moisture in general, and sweat stains in particular, can be very bad for a leather hat. That moisture can soak into the grain of the leather and destroy the appearance and the quality of the hat.
Dip a soft cloth in warm water. Wring the cloth out until it is only damp.
Apply a small amount of baby shampoo to the damp cloth. Use a few drops at first, and add more if necessary. Rub the cloth to work up a good lather.
Clean the sweat stains off the leather hat by rubbing the area gently with the damp cloth and baby shampoo. Wipe the excess cleaning solution away with a damp cloth.
Apply a quality leather conditioner to the entire hat to protect it. Apply a water repellent solution as well to provide extra protection.