For me reading has always been the best way. Read books that have lots of vocabulary. At first you will be constantly looking up words in a translator/dictionary. But soon you will notice that you have to look up less and less words and you will understand more and more.
Use it and use it often... plain and simple.
I am constantly trying to read a lot: books, magazines, internet blogs. And every time I find a new word I repeat it to fix in my mind, and I also write down it on my notebook.