Team trainings are good, or look for sponsor appearances (Telstra store, rebel sport etc).
I prepare my cards for signing with either a light rub of a pencil eraser or a baby powder. They help the sharpie to stick to high gloss surfaces.
Make sure you have your own sharpies and that you’ve tested them before hand.
I used to put the cards in a booklet held by photo corners. That negates the risk of smudging wet sharpie or finger prints.
Sounds obvious, but know your players and use manners. I’ve seen some people ask a player who they are and then flick through their cards trying to find one. Kids always go first. Usually you can get 3-4 “regular players” in the time you want for superstars.