In the IELTS speaking and writing tests, the most important thing is to answer the question. This seems obvious, but many students are so worried about using 'difficult grammar' that they don't give relevant answers.
In the speaking test, listen carefully to the question and answer it as if you were speaking to a friend. Try to speak without thinking too much; forget the grammar and just have a normal conversation.
In the writing test, take some time to understand each question and plan your answer. Check the question every time you write a new idea; make sure that everything you write is relevant. If part of your answer is not related to the topic you will lose marks, even if your grammar is perfect.