As my mentor, Kitosdad, said we should read in order to improve our writing skill. Everything is good for making your writing better. However, because many people in this forum want to take TOEFL exam, and they do not have much time to study for it, it stands to reason that you choose materials which are better for you.
In the first place, it is better to study other members’ essays and their revisions which has been carried out by sir, Kitos. I think learning English is a process which involves a group work. You can learn from other people very easily in this forum. Other people’s mistakes are the things that you should learn. Sometimes, people state their feeling in a special way, but you state the same feelings in another way. By reading other’s sentences you will become equipped with more ways to express your thoughts and feelings. For example, I myself learned a lot of things by reading Leenda’s essays. I used to write some of her sentences, expressions and collocations on a piece of paper.
Second, you should try to study things which are not very complicated as to grammar and topic. I think the essays on this forum have this characteristics. You know, you should read something that is interesting for you. For example, if you are interested in politic, read political materials. This is because you can imagine what the author says, then you are more likely to use his sentences and collocations in your essays.
Third, I think it is a good idea to listen to speakers who speak very clearly and simply. For example, Obama’s speech have this feature. He speaks with passion and word by word. His speech shows how Americans speak formally. In addition, for the TOEFL you are supposed to speak and write formally. So, listening to them can improve your speaking and writing skills.
Fourth, write about different topics. This is because of the experience that I had at this forum. I realized that I am good at writing about specific topics, but I have some weaknesses in writing about some other topics. So, write a lot and write about different topics.
Finally, review your previous mistakes and try not to repeat them.
By the way, some people are so weak at structure that they must study a grammar book.