If you are already at the band-6 level, then you are doing pretty decent. However, if you want to get an even higher score, you should work on your language skills more. I mean, even if you have great strategies for listening and reading tests, and you learned vocabulary chunks to use in speaking and writing, you cannot get 7 or higher without REALLY knowing the English Language. I recommend to listen and read a lot a lot of authentic materials and reach the level where you really understand the conversations of native speakers and you understand most of the articles written for native speakers.
One more thing, you should do practice tests a lot. It really improves your “test” skills. If you want to learn more about IELTS and want to succeed with it, visit TEACH5. Happy studying!