I have heard of polyglots, people who can speak many languages; but now there’s a new word for me, ‘hyperglot’, people who can speak ten or more languages. How do they do it? According to the BBC it’s easy; just ‘inhabit’ the language and culture (BBC, How to learn 30 languages). That means, try to become like the speaker from that place, and temporarily abandon your own mother language and culture. Build friendships with people from the target language / culture. “Resisting the process of reinvention may prevent you from learning another language so well, says Keeley, who is a professor of cross-cultural management at Kyushu Sangyo University in Japan” (BBC). In short, try mimicking people from the target culture is more effective than trying to hold on to your old self.
However, I think that there’s one more thing that’s vitally important: a willingness to try.