A permanent marker is a good thing to have in your bag so that you can easily and quickly write your name and phone number on your discs. There is a good custom that when you find a disc on the track that is not your own, you send an SMS to the owner, or hand it in to the track's lost-and-found. In this way, the disc will be safely returned to the owner, and you will also get your lost discs back more easily.
Sharpie permanent marker is a strong pen that lasts a long time on your discs.