A defined structure with specific milestones. If you can nail down an Inciting Incident by page 10, and a Catalyst by page 25, and a Big Twist by page 60 and your climax happens no later than page 90... then everything else will fall into place.

The odds of success. If Hollywood buys X number of scripts a year, and there are Y number of new scripts registered every year, then your chances of selling a script are Z%. You can influence your percentage chances by getting a manager or an agent, making friends with an executive, etc.

Chasing trends. If you can just catch the right trend at the right moment, you can sell something easily. Once that zombie movie comes out, yours will surely sell. When zombies are done and the next thing comes along, make sure you start writing one of those movies right away so people will buy it. Conversely, if a particular movie tanks, you might as well give up on other scripts in that genre because no one will want them. THE LONE RANGER tanked, so who wants to make a Western?

If you read enough advice books, listen to enough podcasts, go to enough seminars, etc. then eventually you'll develop the skills you need or the connections you want to become successful.

Pay attention to formatting. If you get any part of the formatting wrong, people won't read your script. Make sure you don't improperly capitalize a character's name, or use italics for the wrong reason, or put too much white space or not enough white space on the page. Don't be too wordy, but don't be to sparse. Don't say things like "we see" or include camera directions or tell the costume designer how to do her job by describing the wardrobe the characters wear.

Draw attention to your work with additional materials. Put together a "look-book" with ideas for casting and location choices. Spend the money to create a fake trailer of the eventual movie. Pay someone to put together a budget and a schedule and storyboards so that you can act like you have a full movie project rather than just a script.