Before rushing into writing essays, make sure you have good vocab, build up your vocab by doing a lot of reading and memorising words. This way your vocab in essays will improve and thus improving the standard of your essays.
That's true. You should also try memorising a lot of relevant idioms. Examiners love idioms, with the only proviso that they are used correctly in the right context.

I remember when I my vocabulary really sucked big time and I used to read thoroughly and translate each word, word for word. If I didn't know what it meant, I would look it up and then keep a "Vocabulary Book". In that Vocab book, I would split it up intwo three sections. One section for the word, the next for the Pin Yin, and the final column for the English translation.
If you are going to do what I did, make sure you review the book regularly, or else it will be to no avail.