Firstly, expanding your vocabulary will allow you to be more flexible when writing, however you need to go about learning the vocabulary in an applied way. Make sure you are able to use it in context (this will also make it easier to learn). For example, I could offer a word and you could practice using it in a sentence. To find useful words to learn, reading newspaper articles is very helpful, because the vocabulary is often in a slightly higher register that is appropriate (and impressive) in essays.Secondly, phrases that offer a fixed sentence structure are useful, because they can help trigger more advanced ways of presenting our ideas. For example, 'vorrei dire fin dall'inizio' is not only useful as an introductory signaller, but it also immediately shows a command of the conditional tense. It is crucial that each phrase used, however, is fully understood before use, because we need to maintain flexibility in our writing. Therefore, as a third point, I would advise practicing writing in Italian regularly, even if this means writing a diary entry five minutes each day - the writing does not necessarily have to be academic, but simply in Italian.