I can tell you what not to use; Scriptie permanent marker. We had our Disney Baseball cape style autographed and it got soaked in a rain. It wasn't in a zip loc in the tote bag. When I grabbed it to save it, to late the marker rain all over.
Try fabric pens from a hobby/craft shop. Use them on fabric like you will use and wash it, see what happens.
But, if you are going through the trouble for the autographs, I would do something to frame, like a photo matte you will have forever.