How do I expand my vocabulary and write more sophisticated phrases in my essays?

Read Spanish newspapers and note down any interesting words and test yourself on them (only has to be about 10 a day). Especially good for political/environmental articles as these are subjects which generally come up in essay questions. Also keep up the literature reading alongside, Isabel Allende/Laura Esquivel/Lorca have many accessible books which are enjoyable and hugely expanded my vocab. Reading them alongside the English versions will also aid your translation skills and make you translate sense for sense rather than word for word.