From preparing students for the test, here are a couple of things I suggest you do apart from the usual study:
- read a lot. Students often have problems with collocations and set phrases, so your best best is to see them in use.
- get a list of IELTS practice questions (you can search these online) and find an English speaker with whom to practise them. Just say, "hey, I have an IELTS test soon and I have a handful of practice questions. Could you listen to my answer and give me some feedback?" :) You don't need a native English speaker; just any advanced speaker can help you.
- be super-strict with your writing. Practise writing to the word-limit and make yourself capable of recognising what 250 words of your own writing looks like. A common mistake is to write too little or (worse) too much.
Best of all, you can do a lot of this on italki. ^^