"Subsidize it and you'll get more of it. Tax it and you'll get less of it."
Whenever I hear one of my children using a word that I deem to be above their age level, I recognize and reward that behavior. I am constantly giving out dollars to my kids for great words; this has caused them to be on the lookout for words that will impress dad. It's always fun catching them doing something right.
Some of those dollars come back to me in the form of taxes for toys left out were they shouldn't be. But on the whole they use enough good words to show a net profit.