I like fabric markers. I find them in Michaels or Hobby Lobby by the tshirts and tshirt paint. One brand I can remember is Tee Juice. I cannot remember if it needs to be heat set. They last better than anything else I have used.
I made a label years ago on a friends' quilt with a Sharpie and heat set it...you can still see the ink, but no way you can READ it.