Here is a method I used for a while to improve my already exorbitant vocabulary:
Visit this site once per day, or better still, make it your home page:
http://dictionary.reference.com/wordoftheday/
It has a new word each day, with the full origins (usually Latin) and the meaning of the word as well as examples of use. You don't need to memorise these meanings of by heart. Osmosis comes in here.
Now, the trick comes to actually applying these words when you converse or when you write. It is easier in writing but still not simple. You see, when you read these words, you essentially pick up their meaning and this allows you to udnerstand them when you hear or see them. however, actually utilising them is more difficult. Don't give up, though!