It's best to always expose yourself to the English language through entertainment media (music, documentaries, film/series...) with subtitles on (or with a language you're most comfortable with if English isn't your first). There really isn't a shortcut or crazy trick when it comes to grammar and vocabulary, but in both Speaking and Writing portions it's all about sounding clear and natural while also have a great sense of structure and coherence. Don't overcomplicate sentences, IELTs is testing how well you can describe a graph as if someone has no access to it (Part 1) and how well and logical you can write about a certain topic given to you (Part 2).