I forget where I read it (possibly Robert McKee's 'Story') but there's an idea that you should start your story or screenplay when your character's life changes, follow the conflicts that the change brings, then end the story when the conflicts are resolved.
( tsukimasu posted this in
http://community.wizards.com/bookclub/go/thread/view/110769/29103987/Book_Club_Discussion_-_Brimstone_Angels_-_Prologue__Chapters_1-2?pg=2 )
This sounds like GREAT advice. Like it should be DUH, intuitive, but I've never thought, seen or heard it stated so clearly.