This is the final phase of learning a language, and it is suitable for you when you speak it at about the C1 level. After some time of learning a language, there comes a phase in which you will understand most of what you read, you will know (at least in principle) almost all the necessary grammar, but yet you will be having hard times conjugating some irregular verbs, using correct case endings, recalling the exact word being on the tip of your tongue etc. when being under time pressure (i.e. when you have to speak with other people with a certain degree of fluency).
This is what I call the mastering phase of a language. What should be the next step depends largely on what your goals are; if all you want is a passive understanding of the language, then LingQ is going to be your best friend. Just go through tons of texts in it to improve your vocabulary size and your understanding of spoken language (you should also do some listening exercises with texts you have already worked on; just listen to the recording with your eyes closed, only looking at the text if you did not catch something). Another great tool is the Assimil advanced course; unfortunately, the selection of the advance courses is much more limited than that of the “With ease” series for beginners.
It is also important to hear the language as it is spoken by ordinary native speakers. The authors of recordings in LingQ usually pronounce very carefully, which makes it easier for you to understand, but which also means that you don’t have enough contact with the language as it is normally spoken. A good and entertaining way to fill in this gap is to watch films with subtitles in the original language (it might also be a good idea to import the subtitles to LingQ first, so that you are not distracted by unknown words while watching the film). Some people like to listen to songs in the target language, but I find it pretty much useless, because both the lyrics and the pronunciation are usually unnatural.
If you, however, want to learn the language actively, then finding a native speaker (or someone close enough) is a must have. LingQ and Assimil are still great tools for expanding your vocabulary, but you have to practice speaking with someone who provides you with feedback on what you say wrong. Try to find a tandem language learning partner in your neighbourhood or, if you don’t have such an opportunity, try to find someone on the Internet and speak with him or her through Skype or a similar service. I have found that when someone corrects me while I speak, I tend to remember the correct way with much greater probability than if I just realize myself that something is wrong after consulting a dictionary.