The people who succeed are the ones who have spoken and written their goals for acquiring a second language. I have asked many times, "Why are you wanting to learn English?" General answers like, "To understand someone when he or she talks to me," is usually not enough to learn fluently. But, "my company is paying for me so I can become a supervisor," usually means the person will be successful.
One of the reasons babies learn a language so fast is that they have to learn it. The need to speak may be innate as well, but needing to know what was said is also a survival maneuver. When learning to speak, you really need someone to be able to ask and reply many times so that you can understand and to be patient with you until you do. Then, you will find yourself manipulating ideas, the ingredient to communication.
Retention is often a hard to achieve, partly because a person's interest has to precede learning. But, in language there is a plain and simple principle. You begin your path to fluency the more you think in the langauge you are acquiring. You are really not trying to replace your native language. You are adding to it as a first response so that eventually your mind will differentiate between the two languages in a microsecond and give the appropriate word or expression.