We used to do a 'write better than' so we would get a section of Charles Dickens or any section of a book you like, and try and write it back better. I know it is hard and practically impossible to do but it really helps your vocabulary and as you are working with a really good section, trying to write it back better really stretches your thought and imagination. I found it really helpful anyway. So you could find a book review that is really good and try to write it back better.

Also always make a plan, what you are going to write about in the first section, second section and so on, so you have something to work with. I can never write without a plan. So decide what book you want to write a review for and jot down first of all some thoughts, then put the into points, point one, two and so on, then enhance it and try to write an essay. It doesn't matter if it starts off small, once you have the main idea of what you want to write about down then you can enhance it!

I hope this helps.