There are no special memory tricks that I know of or practice. None of that "memorize stuff for its own sake" stuff (either flash cards or "memory rooms" or whatever).

I remember stuff because it is interesting to me and because it interacts with other things that interest me. It's the connections that make things easy to remember — they aren't isolated facts, they are leaves on a massive tree of knowledge I have been building for many years now.

(And I also remember where to look things up, when I need them. I may not remember every little detail about something, but I can remember, "oh, that book had a section that was relevant, didn't it?" And then I look that up.)

To put it another way: the reason that I remember what appears to be a huge amount of detail about what I study is because it's what I spend my day thinking about. Most people know LOTS of details about their jobs and everyday lives and it's not surprising. Well, this is part of my everyday life and job. And if it's something I teach, well, that just cements it: if I have to come up with a way of explaining something, or telling a story about something, to someone else, and I do it every couple years or so in a formal setting, I'm going to remember it pretty well.

Also, you are only seeing what I remember, not what I've forgotten. So you're getting a pretty one-sided account unless you are also an expert who can see what is being left out. This is why academic conferences and peer review can be a bit tedious ("you left out something that I find interesting, so now I must tell everyone here that!").

The trick isn't remembering facts about history. The trick is making facts about history part of your everyday life. If you do the latter the former comes very naturally.