Motivation and interesting content.
The comments below certainly confirmed the importance of motivation in language learning. It was also pointed out that we need to communicate in the real world. Of course, in an ideal situation we have the opportunity to speak with native speakers regularly. Unfortunately that is not always possible. The alternative is to pay a native speaker to talk to us. That also is not without drawbacks. It can be very expensive. Furthermore, we may not share interests with the native speaker and therefore the conversations can become artificial and uninteresting.
At The Linguist we have loaded up over 70 hours of natural content including a lot of real conversations on a variety of subjects. This allows you to immerse yourself in real meaningful content on subjects of interest.This is still not ideal. You need to talk to people after all. However, once you downloaded something you like listening to, you can take it with and listen over and over again. It is like having a portable immersion class with you. It is surprisingly effective in preparing you for real conversation with native speakers.
Relevant content can be a big motivator. If you are interested in listening and reading, you acquire the language almost as a byproduct of your pursuit of subjects that interest you.