Japanese numero-mnemonics
In "Remember the First 100 Digits of Pi Using This Basic Technique" (mental_floss, 12/11/15), Caitlin Schneider describes a "memory palace" in which one can use letters to recall long strings of numbers.
The second commenter, Helvetica Baskin Robbins, describes a Japanese mnemonic system by means of which one can use numbers to recall sequences of numbers.