Take a dictionary while reading or writing an article. Believe or not, it DOES help. Most people find it annoying to handle dictionary while reading your favourite gossip magazine but focus on the fact that you WANT it and it would be fun to impress classmates with a wide range of vocabulary.
Try to find the meaning of the unusual word you read the instant you find it because if you put it on a longer term it might just slip your mind!
Be yourself! Increasing your vocabulary and being artificial with it are two different things at the same time!