Fill your deck with 150 cards then start to upgrade players. This allows you to only improve your best cards. Begin with the rarest cards and continue towards the more common ones. Maximize them before going to the next player. If you get better cards in the process, discard the ones at the bottom. Be patient and do not purchase cards from the Auction Market. However, you can sell your unwanted rare cards there. Set a selling price slightly below market value to improve the chances that they will sell. Use the profits to improve your remaining cards.