Sometimes the most obvious, most common sense advice is the hardest to grasp. For screenwriters, here is some of the best - and simplest - advice: Think like a producer. From the linked article:
Elements that will get the picture funded are: Cast, Genre and Budget. When these three items are aligned, your screenplay will have a better chance of getting made. Let's take a look at each of these from a producer's perspective.
So simple, and yet so very important. As writers, sometimes it's hard to remember that we also have to look at our work from the perspective of producers, editors and publishers. We get so wrapped up in how important the story is, we sometimes assume it will have the same effect on everyone else. But like everyone else, people in the position to usher our work into the public eye have their own issues to consider.