Clarity is the most important thing to think about when crafting an answer. This is as true of Speaking as it is Writing.
It is a common mistake to think that your IELTS answer needs to be full of long, complicated words. Similarly, it is also a mistake to think a sentence needs to be long and complicated too. Even native speakers don’t think about this when talking. I suspect this is true in any language, ie. ‘Just get to the point!’
For an IELTS student, it is tempting to think that, in order to achieve a higher mark, one must use ‘high level vocabulary’. Worrying about this will likely cause a mistake. Don’t force in vocabulary words that you don’t know how to use properly just for the sake of a higher Lexical Resource mark. It will make ‘Task Achievement’ AND ‘Coherence and Cohesion’ worse. Don’t create the chance for a mistake.
I have talked before about ‘Making a Good Argument’. You cannot make a good argument if your language isn’t clear. So don’t use words that you aren’t absolutely sure about, and don’t make sentences too long. Sometimes, even if you know a more complicated word, it is better to use a simpler one. Making a point is far more important than the complexity of the words. And you want this point to be clear. The structure of your argument will also be clearer if your language is simpler too.
The way to truly take in more complex vocabulary is to read books, but I have said that before. When you read, you will notice that authors don’t make long sentence with unnecessarily complicated words even when they are native speakers.