The way I did this for a specific ordering of cards (used for a set of magic tricks called Mnemonica) was to have some sort of 1 to 1 mapping between each card and its position in the deck.
Some assorted examples:
5 : 4 of Hearts because 4 is 5 minus 1 (and the Hearts are just there).
7 : Ace of Spades because 7 is a lucky number and the Ace of Spades is a lucky card.
8 : 5 of Hearts because 5 looks a little like 8.
49 : 5 of Clubs because 4.9 is almost 5 (and the Clubs are just there).