Instructions
Keep cards in top shape. Only those in mint condition (as they came out of the pack) will retain top value.
Know what your cards are worth so you can be sure to get a fair trade. You don't want to trade a $60 first-edition Gate Guardian straight up for a $25 Dark Magician.
Drop in at comic-book shops and card stores that allow kids to play the game and make trades. Also, the shop owner may take your extra cards in exchange for store credit.
Keep a poker face, no matter how badly you need the card. Some cards are necessary to play combinations. Everyone knows you can't play Great Moth without a Cocoon of Evolution.