Look at your cards and leave them alone.
We have some nervous players who unconsciously bend or twist the cards. While these high strung players do not mean to mark the cards, it is still not pleasant to watch our cards get abused.
If you see a card abuse, just quietly point out the problem to the nervous or bored player. They will stop. There is no need to make a federal case out of the situation. Save your aggression for pot splashers.
In our game, the most damage that occurs to the cards is due to abusive shuffling. Ask Phat_Stack to show you the Riffle Shuffle.