I have seen a lot of good students scoring less than Band 7 in writing while score Band 9 in Listening and Reading.
The reason for low bands in writing is that they focus on the quantity and not the quality of writing. The key tips that have worked for many students to get Band 8 (Part 2) is as follow:
1) Structure: Do not include short sentences, try to use linking words to structure your sentences, stick to a single point in each paragraph and try to use one example in each paragraph of the essay. (This weights arounds 2 Bands)
2) Lexical Resources: Examiners of IELTS have written books on vocabulary and they expect students to correctly use them in their writing exam. Spelling do matter a lot in writing and also the use of a word (This part almost weights 2.5 Bands). Examiners set exam from 24 different topics (L,S,R &W) and therefore, it is better to learn words from each of these topics. There are plenty of material available online.
3) Parpharasing Exercise: Many students who gets Band 6.5 instead of Band 7 fails to recognise that this task even exists. Examiners in Task 2 requires student to begin their essay by paraphrasing the question and failing do so will cost you 0.5 Band.
4) Connecting Words: Examiners wants to look at connecting words sentence after sentence. Such as However, Although, In addition, Firstly, Then, In Conclusion etc. (0.5 Band)
5) Conclude: Examiners require you to conclude your essay and provide clear opinion (Yes or No) if asked in question. (0.5 Band)
These are some of the things that have worked for students in the past. I hope it will help students to improve their scores.
There are a lot more useful tips and material available online that can get you achieve your targets.
Finally, get practice in the following areas for exams in 2017.
1) Environment and Government
2) Transport and Traffic Management
3) Modern Technologies
4) Employment and Economies
5) Public Sector (Health, Education or Law)
Best of Luck!