How mnemonists memorize things
The method of loci is the most useful and efficient memorization technique. It has been used since the antiquity in ancient Greece and ancient Rome, and was a common mental tool in the middle ages. Now, its mainstream use has been lost, and it is mainly used by the so called mnemonists, either to remember useful information or to enter memory competitions.
In the study of memory, in cognitive psychology, it is often said that only 7 plus or minus 2 elements can be memorized in short term memory. However, the performances realized in memory competitions are orders of magnitudes beyond that. For example, the world record for learning digits in an hour long memorization is a staggering 3029 digits: almost 1 digit commited to memory every second for an hour. It does require a huge amount of training to reach such performances, but the vast majority of people can learn to use the method of loci and remember much more than one thought was possible in just days of practice.
How to use the method of loci
If you visualize a mental picture in a mental place (= a “locus”, the latin word for “place”), when you think again of this mental place, you'll retrieve the mental picture. This is the essence of the method.
What is a mental picture?
Anything you visualize in your mind's eye is a mental picture. For example, imagine Batman, standing on your bed, drinking in a huge beer mug, letting beer drip all over. There, you just have made a mental picture. Actually, you may find yourself unable to visualize things in your mind's eye. That's a rare condition, only recently named aphantasia. In that case, you will probably not be able to use the method of loci... sorry!
Where can you place mental pictures?
Anywhere you like! Our brains are very powerful for memorizing the spatial organization of the different places we visit. And we do that unconsciously: we don't need to learn the spatial organization, the brain just remember it. Again, some people have a lot of difficulties with spatial organization, and may be unable to use the method of loci for that reason.
So, in order to use the method of loci to memorize some information, you need two ingredients:
A mental journey. A journey is just a list of loci that you are able to visualize in a specific order. For instance, you might scan your bedroom in the clockwise direction from the point of view of your bed: you first decide to see something on your bed, then you place a second picture on your desk, a third in your wardrobe, and finally a fourth at the window.
If you think about it, you will realize that you know a lot of places. All your friends' houses and appartments, all the restaurants and stores you visit, all buildings where you have some business to do, and even places you've been only once let you some mental mark.A way to transform the information you want to remember in a mental picture. That is the hard part. Remember, if you can picture it, you can remember it. Now, how can one visualize something like the digits “6128”, or a name of an historic figure? The method to use depend on the type of information you want to memorize. Let's distinguish on one side the information that consists of a finite set of elements that recur, and on the other side the information that is arbitrary or whose set of elements is too big.
Creating mental pictures for information from a finite set of elements
If the information consists of a finite set of elements that recur again and again, you need a code that transforms each of these elements into a mental picture. Numbers are very useful to memorize, because you can memorize quantitative data, dates, lists, phone numbers, etc, so the first code you'll want to learn is a code for learning numbers. There are several ways to do it, depending on how much time you have and how proficient you want to become.
Code digit → picture. The easiest code. For instance: 0 can be visualized as an egg, 1 as a stick, 2 as a swan, 3 as handcuffs, 4 as a sail, 5 as a hook, 6 as a cherry, 7 as an axe, 8 as a snowman, 9 as a balloon. Since you have to use a picture for each digit, it quickly gets messy, and it's hard to remember things like phone numbers that way.
Code digit → consonant sounds → words that you can visualize. For example (probably not a great one, I just made it up, mine is in French), 0 can be the sound “z”, 1 the sound “l”, 2 the sound “n”, 3 the sound “m”, 4 the sound “r”, 5 the sound “v”, 6 the sound “k”, 7 the sound “s”, 8 the sound “t”, and 9 the sounds “d”. Then, if you want to remember “6128”, these digits correspond to the sounds k-l-n-t, which could be transformed and remembered as a “CLeaN Toe” for example. You may want to use always the same words to represents combinaisons of 2 digits, so that you won't lose time finding picturable words each time. That's called a major system, read more on that here.
Person-Action system. Quite sophisticated, but very powerful, enable to visualize 4 digits per mental picture. See a description of such a system here.
Other things you can learn with the help of a code: Chinese characters, playing cards, chemical components... Probably much more.
When you can't use a systematic code
When you have no way to use a systematic code, you will need to use your imagination. Sometimes, the information to memorize can be visualized directly: for example, concrete words are easy to picture. If it is not the case, you need to make an association, any association, with something that you can visualize. For example, let's say you want to memorize “Weihnachten”, the german word for Christmas, pronounced “vine-art-en”. You could picture a Christmas tree on which a vine grows, with the Mona Lisa painting (“art”) at the top. You don't need to encode everything, often just a part of the information will be enough to recall all the information. Here you wouldn't have encoded the spelling, but you would probably have encoded the pronunciation.
By combining different information in various encodings, it is possible to create a system of meaning that enables to encode a lot of information. At first it takes some time to come up with the pictures and to develop the system, but with practice, it can become incredibly efficient. Also, “placing” mental pictures is really fast, even at the start.
There is a lot to say on the subject. Depending on what you learn, there are various ways of doing things. My favorite book on the subject is “How to develop a perfect memory” by Dominic O'Brien (he also wrote more recent books, but he focusses less on the techniques, and more on dubious lifestyle things).
If you want to test the method of loci, it's simple: chose a journey fo 10 loci, write down a list of 10 concrete words, and learn it. Then you can learn 20 words in the same 10 loci (by creating a picture combining 2 words in each loci). You should be amazed by how fast you can memorize 20 words in the correct order this way.
Hopefully this article is clear. It's easier when you can demonstrate the method and guide people into using the method. It's very personal, either you like that kind of mental work, or don't. Obviously it's quite nerdy too, but it's a powerful mental tool that can help you save a lot of time if you find an application (I learned the ~2000 japanese kanjis with the help of the method of loci).
I can write more articles on various subjects, maybe specific subjects, if there is some interest for it.