Interesting passage in Rhetorica ad Herennium, from 2,100 years ago:
Now, lest you should perchance regard the memorizing of words either as too difficult or as of too little use, and so rest content with the memorizing of matter, as being easier and more useful, I must advise you why I do not disapprove of memorizing words. I believe that they who wish to do easy things without trouble and toil must previously have been trained in more difficult things. Nor have I included memorization of words to enable us to get verse by rote, but rather as an exercise whereby to strengthen that other kind of memory, the memory of matter, which is of practical use. Thus we may without effort pass from this difficult training to ease in that other memory. In every discipline artistic theory is of little avail without unremitting exercise, but especially in mnemonics theory is almost valueless unless made good by industry, devotion, toil, and care. You can make sure that you have as many backgrounds as possible and that these conform as much as possible to the rules; in placing the images you should exercise every day. While an engrossing preoccupation may often distract us from our other pursuits, from this activity nothing whatever can divert us. Indeed there is never a moment when we do not wish to commit something to memory, and we wish it most of all when our attention is held by business of special importance. So, since a ready memory is a useful thing, you see clearly with what great pains we must strive to acquire so useful a faculty. Once you know its uses you will be able to appreciate this advice.