The easiest way to ensure that you finish your story, and that it has a logical progression, is by deciding on how you intend to finish it. You often hear that stories are written backwards. Not knowing this is like setting out in your car but not knowing where you are going. Why would you want to drive aimlessly through the streets going nowhere? It is the same with your writing. If you don’t know where you are going, you will never arrive, and of course your story will never be sold.