Screenwriting isn't as easy as it seems, and many screenwriters can't make it past the second act of the screenplay. It's called writer's block, and one reason many writers fall victim to this is because of a poorly outlines idea. The only way to make sure you get a completed first draft of your script is to properly outline the whole storyline.
Some like using a pad and pencil. A few choose to outline with the outlining capabilities of Final Draft or Celtx. But most screenwriters end up falling back to a much used outlining process: index cards. But Mike from the SubStream's "Film Lab" has a different twist on the index card plot-builder, and it involves building a pyramid of single-sentenced plot musts. If you're looking to finally get a hold of your script, you have to try this method out.