There are many books out there that offer you tips on becoming a successful, professional screenwriter. And, these are my opinions - being the director of Telluride IndieFest and Key West IndieFest - opinions that I share with aspiring screenwriters on college campuses.
Now, and like Dennis Miller says, "These are my opinions, and I could-be wrong." - however, I doubt it, for these opinions are 'tried and true' that have lead many of our festival entrants onto the 'road to success' as professional screenwriters.
1) Concise, yet clear.
Brevity and clarity are a BIG virtue in screenwriting!
Narrative should be brief, albeit highly-visual for the director, for once your script is sold and/or optioned, the script becomes the domain of the director. After years of previewing the selected scripts for our IndieFest events, one screenwriter I know has become a master of this concept of writing concise, yet clear. Allen Cody Taube from Key West (FL), a 10-time award winner at various film and screenwriting festivals across the nation (including ours).
Allen's narratives (in setting up the scene) are simply right-on.
Brief, and highly-visual - with very few words. Sometimes only one line, and frequently with just a few words. Being a screenwriter and a film director myself, every time I read one of Allen's scripts I can easily visualize what he is conveying in setting the narrative - and that is what a director wants to see.
2) Write only when you are inspired.
If you've got 'writers block', or you don't WANT to write (at that time) - just walk away.
Some highly-regarded writers of screenwriting books espouse the theory of writing everyday, often 'forcing' yourself to get in front of that keyboard and write 'something'. Some even espouse that you should write X number of pages per day. I reckon they are suggesting that you stay 'on top' of your work, and to develop a writing discipline - and that may work for some. However, I disagree with them.
If you are not inspired to write, then don't. If you are at the keyboard and nothing comes to you - just get up and forget about it.
Take a walk, take a drive, take a vacation, make love to your mate - anything to take your mind off your screenwriting!
I have written nine screenplays - two have been optioned, and one has been produced into a movie (a feature film that I directed myself, but we will get to that later), and the one thing I realized during the screenwriting process is that I write better - and often 'binge-write' (for hours, sometimes for days at a time) - when I am inspired to write.
Getting away from the writing (if you are 'blocked') clears your head- and that is a good thing, because ideas then start coming to you when you are not (directly) thinking about it!