Finish each other's sentences! Tell us something and the next person has to finish it. So when you make a new sentence, make sure the next player would be able to play on it.
For example:
Girl 1: .... Last night I drank applejuice...
boy 1: otherwise I wouldn't be able to pee. Tomorrow I'm going to play games...
girl 2: about a fat naked scottsman. I ate breakfast....
boy 2: so John can BBQ me. ...........
etc. etc. You get the point, guys! But don't forget to form a new sentence for the next player!!! And no one or two-word-sentences!