Nations
Since I’m already on the topic of puzzles, if you’re a regular puzzle-solver then I really have to recommend memorizing the entirety of Yakko’s Nations of the World song from Animaniacs. I didn’t do it for puzzles (I did it so I could bust it out at parties)(I have never done this), but boy when the crossword is looking for a country of a certain length and you only know the second letter is a, it really helps to be able to recite a list of them to see if any fit. You know, the brute-force method of solving.
Yes, the song is a bit outdated—you gotta watch out for Zaire in particular—but it’s still a great resource to have in your back pocket.
(Okay honestly I didn’t do it for parties. I did it because of this comic, for some reason.)