One of the hardest aspects of screenwriting, for any writer, is the constant struggle to keep your story on target. Each scene offers new and exciting opportunities to wander off track, explore tangents that don't need to be explored, lay down some of the most earth shattering and thoughtful dialogue ever experienced on screen... you get the point. The tricky thing is, most writers don't even know when they start to get off track. OK, all of us. You're tracking A plots, B and C plots, character growth, sub character growth, humanizing the antagonist and on and on and on. The next thing you know, you're posting questions in screenwriter forums about whether or not it is OK to submit a screenplay that is 152 pages long.
There are many tools a writer has to combating this problem. I'm going to highlight the most surprising method I have found. Start writing short films. They don't even have to be intending for shooting. Just start writing shorts. Why does this work? Well, let me relay my first experience in writing a short. Dustin came to me and said he had some people down in New Mexico he could use to film a short piece and he wanted me to write the script. I was like "No problem, I'll bang it out over the weekend". Why the confidence? My thinking was, "I write feature length, 100 to 120 page scripts. How hard can five pages be?" I'll take a moment to let the laughter die down as the veteran writers in the audience can see where this is going. I sat down that night and started to write my little side project. I wrote the opening and it was awesome. Characters were introduced, snappy banter was thrown around, laughs were had... and I took up three pages of our five page script. Problem. Apparently no matter how short the story, it must still have a beginning, a middle and an end. That's a heck of a lot harder to do in five pages than it is in 105 pages.
How did my little weekend project fare? It turned into a two week passion project. By passion I mean a passion to beat the *goshdarn* thing into submission and show it who the boss was (I'm a little competitive). In the end, I was a better writer for it. It was a very quick lesson in making your writing as lean as possible. In making your story as lean as possible. In making sure you are only saying what you need to say and not taking too long to say it. So if you want to improve your feature writing, I wholeheartedly suggest you write some shorts.