I am a non-native English speaker. When I did my IELTS, my English tutor (who is native) told me IELTS looks at your English EXAM skills. So make sure you do everything correctly, like spelling and grammar. Don't use any word that you are not sure about the spelling. They penalise any spelling and grammar mistakes.
For writing, read some sample articles that scored 8+ to get some ideas how they structure the articles. Follow the structure they required. Use some linker words and give examples on your argument. You can surely find some samples from google.
For reading and listening, for me, just do more practice tests. This two papers have model answers, so just do the practices. I can get 8.5 and 9 easily after some practices.
For speaking, my tutor asked me to avoid using slang, use proper English and make sure it's grammatically correct. (though he said the examiners have higher tolerance on grammatical mistakes on Speaking test than writing test.
Hope this help.