Durak
Keeping track of cards in Durak without a memory trick
With only 36 cards, a player who keeps half an eye on the discards can often name an opponent's last three cards.
Good Durak players are not card counters in the casino sense. They track a few cards that matter and ignore the rest. Thirty-six cards in four suits of nine is a small enough deck for that to work.
Count the trumps
There are nine trumps. One of them is the face-up card under the stock, and you know who will end up with it. Every time a trump is played, count it. By the time the stock runs out you should know how many trumps are left and roughly who holds them.
Watch the aces and kings
Aces and kings of the other suits beat almost everything in their suit. Note when each one is discarded. Once the ace and king of clubs are gone, your queen of clubs is the strongest club left.
Remember what was picked up
When a player picks up, everyone sees the cards they took. This is the most useful thing to remember in the whole game. If the defender picked up two jacks, they will probably attack with jacks soon, and you should be careful about leading a jack at them.
Use the empty stock
When the stock is empty, the hands are fixed. Add the cards you know have been discarded to the ones you saw picked up, and the rest of the deck is in your opponents' hands. In a two-player game this can tell you their exact hand.
Practise against the computer
Playing a few rounds online, where you can pause and think, is a good way to build the habit. The Durak Card Game has computer players and an optional hint mode. Its strategy notes cover what to do with the information once you have it.