There are several ways for you to improve your writing skills. One of the best ways is reading; by reading good materials especially related to academic majors you can understand the sequence of the sentences and paragraphs.
Also there are lots of skills mentioned in different books and websites about writing for TOEFL reading and practicing them will help you either.
And don't forget to build a very good vocabulary knowledge so that you can use it in the test day. Having a good vocabulary choice will help you a great deal too.