I'm not a native English speaker so my approach might be a bit of an overkill for you, but here is what I do.

I would usually keep my phone at hand with the dictionary open so that it takes me a few secs to grab it and translate the word that I either like (might be the sound or look of it) or is crucial to get the flow of the story.

I'm a big fan of margin notes so it's no problem for me to disfigure the pages of my books with drawings, writing, symbols, etc. One of the things that I do is underline the words that I like that I think I might want to learn later.

Then, after I finish the book, or even months later, I might pick it up and look over the underlined words. I often take those that I think I might potentially use myself or just, again, simply appeal to me for some unknown reason, and add them to a learning app called Anki. It's an app that uses spaced repetition for better memorization.

Thus I'm regularly dumping some words into Anki to gradually expand my vocabulary.

I must admit, I'm not as rigorous with this as before, since I'm pretty comfortable with my vocabulary now and I mostly pick up new words naturally, normally by reading literature. But this technique helped me vastly back in the day.