I don't think historians have particularly good memories or regularly use special memory tricks. I certainly don't, anyway. I retain facts in two ways:

First, knowledge is linked. You can think of it as a network or a scaffold. I sometimes think of it as a big skeleton or a tree. The hardest part in learning something new is getting enough of a "base" network in place that adding new things comes easy. So if I was learning about something pretty remote to my knowledge, say, Ancient China, I have a lot of basic work to do. What's the basic setup? How do dynasties work? What part of the country exactly are things taking place in? What's the context in one dynasty, so I can understand how it changes in the next? Etc. Really basic stuff but until I have that down, assimilating a new fact ("during the Song dynasty X happened") is going to be really hard because it's going to be basically "isolated." If I already have a dense network, then it's easy to add something new, because I understand how it relates to other facts and it becomes not a new thing to memorize but a slight variation on something already there. So for me, I know the history of modern physics like the back of my hand in some areas, so when I get some new piece of information ("Planck's work on the black body problem was related to practical problems posed by the German lighting industry"), that assimilates into an existing narrative very easily (I already have a huge amount of deep feel for who Planck was, what the black body problem was, the relationship between German academia and industry in the late 19th century, etc.).

The "narrative" concept in history is very important here — facts are not just data points, they are points in a line of some sort, parts of a story I am telling. Our brains are very bad at individual facts but very good at telling stories, because stories often involve one thing leading to the next in some kind of logical sequence. We have been telling stories, as a species, for a very long time. It appears to be one of the few cultural universals, that we talk about the past through the use of narrative forms (forms that can vary by culture, to be sure), which is just to say, it seems to be something we are wired to do.

Second, I retell the same stories over and over again. Either in my writing, or my teaching, or even on here. Every time I retell a story, I am reinforcing some kind of pathway in the brain (or something like that, neurologically). If I'm telling a story I haven't told before, obviously I have to build a new one from scratch — that's the experience of lecturing on a new subject, for example. But if it's one I've told before, even if it's one that I haven't thought about for a decade, I am reinforcing something already in place. Sometimes this requires re-looking up a lot — thank goodness for books, articles, and notes from grad school! But the more times I retell something, the easier it gets. And of course if I'm lecturing or writing I can "cheat"; I do refer to notes, and my lecture slides usually have a few things that are hard to recall (I can't remember numbers for the life of me) there so I use them as I wish.

For some areas of history, the parts I end up telling and retelling the same stories over and over and over again (for example, in my case, things related to the early history of nuclear weapons), I just know these things through repetition of the stories. I read and write and talk about the topics with the regularity that most people do whatever it is they do at their day jobs. You don't have to memorize what you spend every day doing — your brain just learns it. So for a very limited subset of history I can do very detailed recollection (exactly what time of what day something happened), but that's no trick, that's just because I have spent a lot of time writing and rewriting and retelling that information, so it's burned in quite deep at this point.

Broad ideas are the place everyone starts. It's always a struggle to learn big new things, and history is a mix of massive narratives and little details. There's no magic way to do it, though immediately writing down the narrative and the importance of the details in your own words (and I find writing down with a pen and paper, not typing, works better at this, but maybe this is just because we didn't use laptops when I was in grade school or college, though there is some research data that suggests that the more senses you actively engage in making a memory, the better your retention is) is in my experience the best way to retain it. Don't think of them as facts to learn, think of them as elements of a story you are going to tell, and then tell it, either to yourself, to others, to a file of notes, whatever. If you do that your brain will reward you. It won't remember everything (at least mine doesn't), but it'll set up something that you can later come back to and re-activate, either to tell that story again, or to add or modify it as you get something new to add to the mix.

I'm not sure if this question is appropriatr for this sub but I want to answer anyway. Please note that this entirely personal experience.

I'm known as somewhat of a 'fact hoarder' among people I know. This has come naturally to me for most of my live but recently (since I've been studying history mostly) I've been thinking how this memory proces works. Especially when I relate it to names.

You see, I'm absolutely terrible at remembering names of people. I've asked people their names 5 times and still forgot it two days later. I have the same problem with celebrities and band names. I know who they are and everything related to them but often I don't know their name. BUT I don't have this problem with historical figures or fictional characters in novels. Those I can name any second.

My gues is that it has everything to do with mental connections between different subjects. Every relates to each other and that is how I remember. 'Rome' connects to Ancient Rome, the Catholic Curch, Italy, buildings like he colloseum, recent socces news, etc. And everyone of those subjexts relates to a whole other system of related subjects, like hooligans destroying a fountain in Rome, which reminds me off their soccer team which reminds of their rival, etc. So it's all about a web of information and placing new subjects in that web.

This is exactly why I can't remmeber names. People tell me their name once but it doesn't come up after that in my interaction with them so I often don't connect it to anything and I forget it.

I hope that was at least somewhat insightful.

I've studied Chinese which requires a lot of memorisation. We all used spaced repetition systems such as Anki (basically, digital flashcards + a learning algorithm) to retain all that information. I know that people studying medicine also use sometimes use an SRS to learn anatomy. It can be used with any kind of information that you need to memorise.