This is a million dollar question! To really understand what you require for band 7 and 8 writing you can look at the Writing Assessment Criteria (public version) and study the criteria. You can download these here.
In my experience these are only of limited use to students as you need to interpret what they mean and it doesn’t matter what study you do online unless you actually write and have your writing checked then progress will be quite slow. The very quickest way to improve writing is to get someone to check your writing and explain how to improve it and the best person to do this would be an IELTS teacher. So, if you spend money on any part of your IELTS training this is the best place to do it.
The second thing you can study to understand the level you require in the exam is model tasks. If you are looking for band 7 or 8 in the exam then I suggest you concentrate on models written by teachers and examiners (such as those in the Cambridge Test Books). Some of these may be a little long but don’t worry you won’t be writing that much – just focus on the vocabulary and sentence structures. Sometimes the student writing at band 7 and 8 can also give useful information if they have been annotated by the examiners.
Finally, make sure you understand and know how to write such things as complex sentences, noun phrases and different types of sentence structures. Then make sure you know the vocabulary you will need to get the right style for your tasks (check the models). Your last job is to get together some lists of topic-related vocabulary and to find synonyms for words that you may want to use a lot. You can find more information about how to improve your vocabulary on my blog.
If you have spent some time on the point above then you should be ready to start your practice. The first thing to do is to start learning how to plan your writing. For Task 1 Academic this means getting good at analysing the data and deciding exactly what you will write about, how you will group things according to what you see and finally what you will put in each paragraph. For Task 1 General Training (the letter), you need to decide firstly if the letter is formal, informal, or semi-formal – this will tell you the style and also the kind of vocabulary you need to use. After this you can decide what outcome you expect to get from your letter and then construct the letter to get that outcome. Make sure you cover the points in the question and then decide what will go in each paragraph.
Task 2 requires the same planning process for Academic and General Training – but it is vital that you do a good plan. Planning your task well has 3 benefits:
By focusing on language you will not be distracted by the topic and your ideas. Your ideas will have been explored and put into your plan. This means that you can avoid all the issues that stop you getting band 7/8 such as repetition of words and structures, going off the point, and lack of balance. This makes sure you have a good range of linking words and making sure that your sentences are all related to each other thereby giving good style to your writing.
If you have a good plan then you are going to expand your ideas in the plan into complete sentences. A good plan will act like an outline which you then need to ‘fill-in’ with vocabulary and structures this means you don’t have to stop and think about where you essay is going your plan will tell you.
Finally you can eliminate errors because you are focusing on the language rather than the ideas and as such you will be aware of things like articles, agreement and the other common errors as you write – focus on language will help you to be aware of these. It also means that you should have a few minutes at the end to check your writing and this means that you can get rid of errors before your script goes to the examiner.
Make sure your plan has some information for each sentence you will write – these will all have a function. Here is an example of the plan for the first body paragraph:
1st Body Paragraph
You can expand these points adding appropriate linking words to join them and a range of sentence structures and vocabulary to both present your ideas AND show your language. This would add up to around 100 words. You then do the same for the second paragraph – change it a little by not including an example and certainly use very different language and then you’ll have another 100 words so you then have 50 – 80 words for your introduction and conclusion (no more than 3 sentences each).
Now you can see how a good plan will almost write the task for you.
Now write your task, check it and send it to your teacher. If you don’t have a teacher (and I advise you to find someone) then leave the piece for a day or two and then go over it again and see if you can discover any mistakes you missed the first time. This is the problem with exam writing, you don’t have enough time to see all of your mistakes which is why you need to be focused and have a strategy to find them quickly.
The best strategy to help you check your work effectively is to have a ‘mental check list’ of all the mistakes you generally make. You will probably recognise these if they are pointed out to you. If you have a good IELTS teacher they will tell you about all of these and you can then make a list and try to avoid them as you write and then after you have finished you can use the list for you final check. The list will not be very long (if it is then you are probably not yet ready to write at band 7/8) maybe 3 or 4 items – you will know what they are for you but in general: articles, subject/verb agreement, singular/plural, prepositions, tenses are all mistakes that are common in IELTS – even at high bands. There may be some other specific ones for you so add these to the list. Don’t forget spelling and punctuation if you make mistakes with these.
Keep focusing on these as you write and you will soon become very aware about the dangers of making mistakes in these areas and eventually these will become less of a problem and in time some of them may disappear altogether. Vigilance is key.
However, getting someone to check your work will make a big difference as it’s almost impossible to discover everything that may be wrong by yourself.
I’m not only a native speaker but also a very experienced English language teacher and IELTS teacher and I always get someone to check and proof read my writing before I publish it. I will get this checked before I post it on my blog.