As I've said before, there is no 'secret' way to improve your IELTS score. You just need to study hard and keep practising. However, if I had to give one piece of advice, it would be this: work on vocabulary.
If you use a wide range of vocabulary in the speaking and writing exams, you are more likely to get a high score. Vocabulary is also the key to finding the answers in the reading test and understanding the speakers in the listening test.
So, are you writing new words and phrases in a notebook every day? Do you read something in English every day? Do you use a dictionary? Do you use Google or Wikipedia to look up words and phrases? If you don't do these things, start now!