The Talmud, in Shabbos 104a says “Make yourself simanim [signs – as study and memory aids] in the Torah and thereby acquire it.
Among the best-known of such devices are:
Gematraios, or numeric equivalents
Rashei v’sofei teivot – acronyms and end letters
Seder AlefBet – alphabetical acrostics
M’shalim – proverbs, parable, allegories
Notarikon or contractions
Simana – catchwords
Grouping and totaling of concepts