Read a Dictionary. i did so all the way back in 5th grade and have never had a problem when understanding people with very advanced vocabularies. of course, some people think that the task of reading a dictionary is quite a daunting task. however, if you were to only read 5 pages a day until you finished reading, perhaps before you go to sleep or during break time, you would find that it is not actually that difficult to do.
If you read a lot, try reading more advanced books. I read this book one time, I had no idea what half the words were, so I was constantly going from dictionary to book, and back again. Afterwards, I knew loads of words and yeah. . .
Use a word for the day. I googled "word for the day" or "word of the day" and the first three hits were websites that offer a new word every day, give its definition, pronounce it, and use it in a sentence.