Screenwriter Scott Meyers suggests boiling the story of your screenplay down into six words. If you can't do that, he says, you don't have a story.
Sometimes I really resist the idea that everything should be simple, but he has a point. Obviously, six words will not encapsulate the totality of your script but especially if you tend to get lost in your story sometimes, this can be a good tool.
Let's try some examples. See if you can tell right away which films I'm describing:
Son takes over family business: the mafia
Racist driver. Black passenger. Southern roadtrip. (I may be cheating, road trip probably should be two words.)
He's down, she's up: love possible?
Or, for a classic play: Father murdered. Son's revenge consumes all.
I imagine you got those: The Godfather trilogy, Green Book, A Star is Born, Hamlet.
Admittedly, these descriptions don't reveal that Michael doesn't want to be the new godfather, or that a friendship develops between the driver and the passenger, or that the 'down' and 'up' refer to fame and musical careers. And six words can't do justice to Hamlet. However, these do all identify the core elements of each plot.
Someone said that plot is what the story is about, and theme is what the story is REALLY about.
For instance, Macbeth's plot is the series of murders the Macbeths commit to try to make him king; the theme is the price of ruthless ambition.
Usually, your six words will be about the plot. In the example above, only the six words about Hamlet hint at the theme: that revenge can destroy the person exacting it as well as that person's target.
Don't try to make the six words reflect the theme unless that comes naturally. Of course, it's your execution of the plot that largely determines the quality of the script, but you have to have a solid plot before you can execute it.
HOW TO USE THE SIX-WORD STRATEGY
I'd never use the six-word format in a pitch, but it can be useful to work it out to make sure you know the core of the story.
Write the six words on an index card and put it where you can see it as you work on the script.
When you're unsure about a scene, ask yourself whether it is in alignment with that core idea. Even if it's a scene in a subplot, probably it will reflect the main idea in some way. I haven't done an analysis, but I'm pretty sure almost every scene in A Star is Born is about either him going downhill, her rising to fame, and/or their love.
Think about the films you've seen most recently. How would you sum up their plots in six words? Practicing with other people's stories will make it easier to do it with your own.