Hi...
I too found it bit tough for lack of practice and still luckily managed 7.5 ....
I suggest you go through the Cambridge series practice tests and prepare with many topics for writing...having a plan if important...test 2 carries 6 points out of 9 and it is always better to write it with a structure...an intro about the topic/ the arguments for and against or pros and cons as the case may be/ the conclusion.
Practice with as many topics in your mind so that at the test,you'll not be surprised with whatever topic and your answer will look organized ...and do practice by writing as it will help pick up speed...if you manage to write well over 250 words and that might help...around 300 words...
you can work with imaginary topics and try developing answers in your mind on each of them...so at the test also you'll have no problem having good flow of ideas /points to fill up ur answer sheet in an organized manner...
http://ieltsonlinetests.com/catalogue