What can I do to get high marks in IELTS?

Practising speaking with foreigners before the test. There is a limited number of possible questions that could come up. They write new items regularly, but they tend to follow a similar theme and stay around for quite a while. So you can get questions set online.

You will be asked about yourself and your likes and interests. Make sure you give extended answers to each question. The later sections require you to talk more and for longer. The last question is always a great question written on a card to read, and you have a minute or so to prepare before answering. It is essential to make good use of that minute to prepare.

Some questions might make you think of something that you haven't considered before or ask you about something you haven't prepared for. To give an example: my students often get stumped by the standard questions, "Describe your favourite ..." It might be a book or a movie or a piece of music.

Hope the help of this tip

IELTS tests your ability in English, hence it would significantly help you increase your grade only if you have intent and time to practice and work on areas that you are weak at.
If a student has no reading habit or has a weak vocabulary, at least 3 months of intensive one-on-one training is needed as opposed to someone who has a strong command of the language.
So, you have first to self-evaluate and understand how much time you need to practice (you can take free mock tests online to see where you stand), see if you need a trainer/coach to help you identify your weaknesses and then work accordingly.
Also, it is a myth that one can be useful in speaking and wrong in other types of IELTS tests. If you are right in reading & writing, automatically it would translate in your speaking and listening abilities, but the vice versa would not work.
I hope this helps. All the very best!