To extend your vocabulary, look up stuff in a thesaurus to find creative new ways of using the search engine. You'll be amazed how many threads like this have already been posted.
Seriously though, the best way is to summarise all your texts then do practice essays in all the different formats you might get asked for (feature article, speech, etc.) If this seems hard, do a straight essay first then adapt the content to suit a different type.
For vocab, learn your essay terminology really well. Understand the difference between portray, convey, illustrate, etc. (cause they all have different connotations)
Use the microsoft word thesaurus: right click on a word and look up synonyms. Make sure you get the right synonym for the context
Never ever ever use 'good' 'bad' 'things' etc. These words are the lowest of low in the english language. They are too general. Use other words such as 'appropriate,' 'effective,' etc. This will make your language more sophisticated.