Recently, Pixar story artist Emma Coats tweeted a series of tips for story development and screenwriting, and they are profound. Tip #8 is something that every filmmaker should know and practice: "Finish your story, let go even if it's not perfect. In an ideal world you have both, but move on. Do better next time." Here are a few others that I like...
Tip #5 is something I harp on a lot: "Simplify. Focus. Combine characters. Hop over detours. You'll feel like you're losing valuable stuff but it sets you free." I like to say it simply as "Kill your babies," or remove the things that you personally are in love with, but do not really serve the story.
And #11 is another one for people who seem to be perpetually waiting for the right time to do their project..."Putting it on paper lets you start fixing it. If it stays in your head, a perfect idea, you'll never share it with anyone."
There are plenty of other tips to learn from, so hit that link above and enjoy.