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Use it. Start off by taking classes where you are immersed in the language, but then you need to use it as often as possible for everything.1
The easiest way is to be around the people who speak it on a daily basis.
I want to learn Spanish
Visit the country of the language you want to learn and stay there up to 6 months or so.
Duolingo is really great, but if the language you want to learn isn't on there or you just want to get into more advanced territory, an actual class will be the most effective.