Don't repeat yourself or the same ideas. This gives a bad impression and the examiner realises that it isn't adding to the content of your report. If you are weak at English grammar, try to use short sentences. This allows you to control the grammar and the meaning of your writing much more easily and contributes to a better cohesion and coherence mark. It's much easier to make things clear in a foreign language if you keep your sentences short!

　　As I just said, if you have finished the exam with time to spare, DON'T just sit there!! Check what you have done. If you have time after the check, check again. And so on....

　　Don't be irrelevant. Although you can use your imagination to expand on your answer, if any part of your report is totally unrelated to the question and put in to just put up the word count, then the examiner will not take it into account and deduct it from the word count.

　　If you want to improve, there's no secret. Practice. Practice. Practice. You won't get better sitting and doing nothing. Even good English users need practice for the IELTS exam. It could make all the difference between your getting the band that you need, and getting half a band less than you need and having to wait 3 months to do the exam again.