The IELTS test evaluates your current level of English and how well you can use the language in the four basic skills. Memorizing complex grammatical structures and academic vocabulary and reproducing them without being competent enough is not the best idea if you want to achieve your highest score. This is specifically true for the writing tasks.
Writing Task 1
Writing task 1 is not a platform for you to write down what you think and to show how smart and knowledgeable you are about a topic. It simply tests if you can properly express yourself in a factual manner using information that is given to you. For this task it is absolutely necessary to practice with fixed structures and learn the language of describing graphs, data, charts, etc. This is something you will need to look into, no matter how good your level of English is. Advanced students often make the mistake of going unprepared into the IELTS test and being surprised about their low scores in the writing section. The writing section is not only about your English skills and how competent you are in writing, it is also about how familiar you are with what is expected of you (so make sure you check the writing descriptors before you take the test).
For the writing task 1, learn the language of description and apply it to your writing. The best material for the writing task 1 which I also use with my students and can recommend to anyone is the IELTS Buddy ebook for writing task 1. It provides you with 'templates' that can be used with any topic and if you practice these templates, you will end up finding the task quite easy.
Writing Task 2
For writing task 2, I always tell my students to go ahead and to try out using academic vocabulary and complex grammatical structures they are unsure of during the lessons. This is how you learn and get better, i.e. by making mistakes and learning from them.
However, if you take the actual IELTS test, there is no room for such experiments. If you write at your current level of English without making any mistakes you will receive a higher score than if you use highly complex structures and academic vocabulary but you don’t use them correctly. So keep in mind to stick to what you are capable of for the writing task 2 .
Practicing for the writing task 2 is similar to writing task 1 as it is best to have a standardized template that you can use for any essay. The examiners marking your writing task 2 do not judge the quality of your arguments, their scores are purely based on the 4 areas: task achievement, coherence and cohesion, lexical resources, grammatical range and accuracy. This means that it is more important how you write your essay than what you write. Of course having great arguments helps but if you cannot put them into English, it won't get you a high score! So don't waste your time trying to find the perfect arguments in answering a writing task 2 question. Good arguments are good enough! Rather focus on making your answer flow and be careful to proofread so you don't make any mistakes.
IELTS Academic Writing Band Descriptors
Find the official writing band descriptor for writing task 1 and 2 here:
IELTS Academic Writing Task 1
IELTS Academic Writing Task 2