Be passionate about the cultures of the countries that speak this language, and read the products of these cultures - articles and books about their peoples, histories, traditions, world views, current events - in that language. This is how I learned English - from the age of 17 onwards I've been passionate about Anglo-Saxon cultures, histories and societies (in particular about the United States) and have widely read about them, in english.

I'll grant you that this is easier if the language you want to learn is spoken in a large, complex country or countries - such as the English, Mandarin, or Spanish.