Water and shampoo, just give it a good scrub and fluff it up with a hairbrush again when dry.
I solved that problem by taking the horrid chunky noseband off, replacing it with a plain strap (roundings actually, perfect for my horses little noses) and using a sheepskin sleeve on top. Just chucked the sleeve away when it was dirty, you can pick up sheepskin jackets to chop up in any charity shop for a fiver...unless they are back in fashion again!