ST 2-2.49.1 ad 2, my rough translation; the Dominican Fathers translation is here. Of course, these are relevant to more than just memorizing things; they are, to take just one example, part of how we become thoughtful (i.e., keep important things in mind), including being mindful of God and our fellow human beings. The references to Cicero are actually to the Rhetorica ad Herennium, perhaps the most influential rhetorical handbook of all time, which is no longer attributed to Cicero. Sollicitudo, here translated as 'care', is one of the major acts of the virtue of prudence; it could also be translated as 'vigilance', 'watchfulness', or, for that matter, 'solicitude'; it is associated with ingenuity and alertness.
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To the second it must be said that just as we have an aptitude for prudence from our nature, but its completion comes through practice or grace, as Tully says in his Rhetoric, so too memory not only derives from nature, but is made greater by art or diligence (industriae).
And there are four ways by which a man progresses in remembering well. The first of which is that he who wishes to remember something should take some likeness appropriate to it, but not wholly appropriate, because that which is inappropriate is more wondered-at, and so engages the soul more, and more vehemently. And this is why we see that in childhood we remember more of what we saw. Now the need for discovering these likenesses or images is because simple and intellectual dispositions (intentiones simplices et spirituales) easily escape the soul unless they are bound to some corporeal likeness, because human cognition has greater power over sensibles. Therefore the memorative [power] is placed in the sensitive part.
Secondly, whatever a man wants to remember he ought to hold in his attention (consideratione) and organize (ordinate disponat) so that he may proceed easily from one memory to another. Thus the Philosopher says, in the book De Mem., From a commonplace (a locis) we seem to remember something, because we go swiftly from one to the other.
Thirdly, a man ought to have care (sollicitudinem) and concern (affectum) about the things he wants to remember, because the more something makes an impression on the soul, the less it escapes it. Thus Tully says in his Rhetoric that care (sollicitudo) conserves the features of representations (simulacrorum figuras) whole.
Fourthly, one ought to meditate frequently on what one wants to remember. Thus the Philosopher says in the book De Mem. that meditations save memory, because, as is said in the same book, custom is like nature. Thus what we often apprehend (intelligimus) we quickly remember, as it were proceeding by natural order from one to the other.
