You'll need a writing buddy for this one:
Make a list of ten nouns, ten verbs, ten adjectives, and ten names. Now separate the words and put them into a cup (or four different cups if you want to keep the categories separated). Now draw four words (or one from each cup), set a timer for ten minutes, and write until the timer goes off. Make sure to include your words!
When the time's up, switch stories with your writing buddy, reset the timer, and add onto his story until it goes off again.
Can't find a writing partner? Set the timer for five minutes instead of ten. When it goes off, draw a new word, work it into your story, and keep going!