There’s a product called Elbow grease in a yellow bottle. Can pick it up in most stores. Does wonders on my hat and polos

The newer styles that are polyester instead of cotton clean up pretty easily. I use liquid dish soap and a soft brush. Wash them out in the sink and hang to dry. They come out clean and hold their shape. I use the same technique on the cotton ones but they shrink a bit as they dry so they never look quite the same. Hope this helps.

Warm water and some washing liquid then make sure you rest it on something to kee shape while it dries

Immediately after playing I try and give them a good rinse through with cold water straight from a tap. Hopefully this flushes out all the salts which leave the stain.
Then into a ziplock bag whilst travelling home. Then hang on a line to dry naturally.
Never found any product that removes stains once they are dried in.
I sometimes put them in a pillowcase and gentle wash in a machine, never had a problem, but it won’t remove sweat stains.
Some spray the inner headband with waterproofer.

Warm water and a little baby shampoo once I get home from my round always does the trick for me.