My (not very immediate) reaction to this was to start listing all the words I know on a computer. This was incredibly boring, time consuming and didn't prove anything. (In hindsight: I could just have started with an e-book, sorted the words in alphabetical order and remove the ones I don't know the meaning of.)
So I have now decided to extend my active vocabulary. Write down some expressions I like, "cool" (in my opinion) words and words that are synonyms to those I use frequently and... USE THESE INSTEAD. I give myself a week to create a list (which I this instant don't know how long it will be), and then I will carry it with me for support when I am about to speak.
I'll let you know next week how it goes!