The biggest enemies of felt are moths, so protection from them is the top priority. If you don't wear it for a long time, put next to lavender or mothball. If moths do start damaging it, putting the piece of clothing in the freezer for a day will get rid of them without damaging the material. If it does get too dirty after many years of wear, it is possible to clean it with a damp cloth, or if that is not enough, wash only by hand in lukewarm water. After washing, squeeze the water out, pull back to shape, and air dry it lying flat.