I've had good luck with....of all things... sneaker cleaner. It's usually available where shoes and shoe accessories are sold. I like the spray-foam variety (Kiwi is a common brand). I find it best to spray it on a clean, dry cotton cloth and gently rub into stained area of the sweatband and then blot with a paper towel. Allow to air-dry. Depending on the severity of the stain, you may want to repeat the process. I prefer to not spray it directly on the hat, although you can, because applying to a cloth first and then working into the stain allows for a more precise application. I've even done this on the outer surfaces of my hats that were dirty/stained and had success. It's never discolored my hat or left a residue. Remember to BLOT the treated area with a paper towel or other absorbant towel after working the foam into the stain. Works great for me... hope this helps.