I start with one sentence, just to get things going, and after that each person posts the whole story so far and then, in bold, the write the next word. No double posting, and no changing the story! Once it's been written, that's how it is!
Example: P1: There once was a red-headed boy. One P2: There once was a red-headed boy. One day, P3: There once was a red-headed boy. One day, a P4: There once was a red-headed boy. One day, a cat