I have an uncle who speaks 6 different languages; he really likes picking up new ones. I once had a discussion about how he could pick them up so quickly and he basically said what the parent was saying: use it, practice it, read newspapers in it, speak to people that speak it too. Immerse yourself, accept that you'll be uncomfortable for a while, and before long, you'll be able to hold a simple conversation.

He also said you know you are fluent the day you can casually joke in real time with people and make them laugh (at the joke!)