Having what is known as “card memory” is extremely important in the game of gin rummy because as the play progresses you are going to be in dire straights if you don’t remember a certain card has already been played if you are waiting on that card. A few fortunate card players are lucky enough to have a photographic mind as well as a retentive memory, but most players have to really work at developing card memory. There is not exactly a system for remembering the cards, but it is simply a matter of training by constant practice.
At the start of a hand, you should try to visualize the 52 cards in the deck, and then deduct the 10 cards that you are holding. As you or your opponent discard cards in turn, you should eliminate these cards from the pictured deck in your mind. After the initial deal, and by the time 1/3 of the stock has been used in play, you should have a fairly accurate picture of the type of cards remaining in the deck. You should also know the type of cards which your opponent can and cannot use.
It is important to remember specifically the cards that you discarded. He will normally pick 1 to 3 of these cards, and they should not be difficult to remember as you had them first. You should keep reviewing the cards he picked up while noting at the same time which cards he is discarding. This will allow you to calculate correctly which cards your opponent is holding. In time this will come easier to you, but for now you need to practice at not only remembering this, but you need to develop a system for general recollection of early discards even if they hold no meaning to you at the time. Near the end of the game, you may need to reorganize your hand based on the earlier discards and if you don’t remember them then you won’t be in a good position to win the game.
Many players have a habit of helping their memory during the play of a hand by repeating the cards to themselves that their opponent is holding. This doesn’t necessarily always work though, because if a player is tired or has seen many hands up to this point, it may confuse them on the next hand. For example if you are repeating a certain run that you are sure your opponent is holding, the following game your mind may keep repeating the certain run, giving you a disadvantage because you think he has different cards then he really does. Then when you pick up the cards or if you see the cards, it may confuse you even more. In other words, one of the most important parts of a card memory is the ability to forget a hand once it is completed.