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Immersion, but since that isn't feasible for most people, traditional textbooks with a teacher, Mango Languages or similar software, consuming media in the target language, etc. And ideally in combination with each other so you're doing a bit of reading, listening, and speaking.

Anki with self-made cards worked best for me.

If your target language happens to be Japanese or Chinese, feel free to checkout my blog https://dayjaby.wordpress.com

Babble helped me quite a bit with my french