Answer by Alex Lightman:
By asking the right questions (you are off to a good start by asking this particular question), because your quality of life is directly related to the quality, quantity and variety of your questions.
By reading and writing about intelligence, learning, game theory, pattern recognition, military strategy and anything that involves complex and/or nondeterministic systems.
By reading novels, especially science fiction series that require keeping hundreds of characters and technologies and weird names in your brain (yes, being a fan of Star Trek, Star Wars, and other Type II and up civilizations on the Karadashev Scale counts, but this works better if you go for the books, atlases, graphic novels, etc.
Speaking of graphic novels…
By reading at least one graphic novel a week for the rest of your life. Easiest way to get smarter. Counts more if you can read it in a different language, even if its the same graphic novel.
Speaking of other languages…
By taking at least five hours a week to always be learning a new language that you are not yet fluent in. I know a guy who runs a solar lobbying organization who speaks Polish, Ukrainian, German, French, Italian, English, Spanish, and is now learning Mandarin. And when I say he is learning Mandarin, I mean…he is studying it AND he has a hot girlfriend who only speaks Mandarin that he chats with on WeChat, with translates back and forth into English and Mandarin, so he is learning as he is flirting. He has had girlfriends who were native speakers of all these languages.
And that's smart.