Try to learn one new word a day. This may sound easy but rest assured it takes a lot of discipline.
This is the inescapable reality to learning new words: to improve your vocabulary (and by extension your writing) you must read more. It is imperative that you read books that engage your brain. Aim for high-quality works and leave gossip fodder to the tabloids.
Furthermore, to truly retain your daily new word it is necessary to learn how to use it in the right context. By genuinely increasing your vocabulary you will not run the risk of sounding phony or contrived in any of your writings.