Immersion, unfortunately.

You put yourself in a situation where nobody understands you in your native language, nothing helpful for your daily life is written in your native language, and you must constantly try to comprehend and produce the target language if you want to accomplish even basic tasks.

If you can't up and move, then studying a language with a lot of cultural export helps a bit, if you can get into the habit of switching all of your media consumption to the target language. It's not the same, though.