Memorizing an entire deck is very difficultand requires practice, practice and practice. ( I tried...)
But, you can fake with simple sleights that you remember the entire deck order.
I usually use the double key card principle and the crimp.
Let me explain myself.
For the double key card, memorize the top and bottom card of the deck (after the spctator's shuffle). A card is selected. Then cut the cards in half and ask the spectator to replace his card on the top packet and you then place the bottom packet on top. The situation now is : the selected card is in the middle of the deck sandwiched between two known cards. You can now allow the spectator to make one riffle shuffle (only one). You then take the deck and ribbon spread it face up. Pretend to memorize the deck while you are only memorizing the cards in between the two known cards (in those cards, there is the selected card). Leave the deck on the table and ask the spectator to move their card (their card only) to any location they want in the deck (during this moment, you repeat in your head the names of the cards in between the two known cards). After this, you ribbon spread the deck again and spot the missing card (one of those in between the two knwn cards). You then locate the selected card. The spectator will think you memorised the deck and you detected the changing location of the selected card.
The second trick uses a crimp. A card is selected by the spectator, you then execute a diagonal insertion, the card will protrude from the deck. This permits you to apply dowward pressure on the protruding part of the selected card. You successfully crimped the selected card! You can now allow your spectator to shuffle as much as they want.
Now comes the "hard" part. Take the deck face up (slightly angled so the face of the pack is facing the spectator's eye), pretend tomemorize the order of the cards and start to count the cards (silently, not out loud) by reversing the order (one for the bottom card, two for the card second from the bottom, etc...) . You will easily locate the crimped card because of the angle of the pack. When you reach the crimped card, memorize its position in the deck (the number you lend on when you reach the crimped card). Finish to count the cards. TTurn the deck over (face down). You ask the spectator to tell the name of his card, you (pretend to) think, and you reveal the position of the selected card. Count down to the number and reveal the selected card.
I hope you understood my unclear explanation, PM me for more details.
I know this is a bit out of subject but these tricks have a great effect on audiences
The first card trick requires a table for the spread but the second one could be done in the hand (of the magician and the spectator).