To the OP,
Some extra tips aside from the ones mentioned:
- read in general: newspapers, novels, blogs, top essays... basically anything you can get your hands on
- write anything, whether it be a story, a to do list, an essay, a letter
- find synonyms to words you tend to use a lot: Find a few old essays and jot down the words that appear to be commonly used between them (e.g. presents) and find synonyms. Use your trusty friend for English (your dictionary) and its friend (the thesaurus) to fully discover their synonym's meanings and how you'd use them in context. Now use this new knowledge!
- Jot down words you know are sophisticated and see how you could use them.
- Sign up to dictionary.com's Word of the Day posts and/or check it out every day (or something equivalent). You might learn a useful word or two. (If you liked this, try to play some vocabulary games on pretty much any website that offers word games. Not the coolest idea, but they get the job done.)