Movie Making Manual/Writing/Rewriting
Over 80% of your time on any screenplay will be spent on rewrites. So much of screenwriting is about structure that you may have the story in the first few weeks while the rest of the time is spent reworking the script so the story works on the screen.
After you have plowed through and have a first draft script of 90-130 pages the best thing to do probably is to get a little distance from the screenplay so you can evaluate it objectively. This is probably best done by laying the script aside for a few weeks and doing something else then returning to it with fresh eyes.
Top-Down Rewriting 
Beginning writers assume that the rewriting process consists of going through the script line by line and fixing any typos or grammatical errors. After a couple of such passes they're bored and the screenplay is still weak.
To be efficient and really make progress requires being more organized and starting with the big picture. Make a series of passes through your screenplay, each time concentrating on one or two elements.
Start with the most important elements of any story such as: Is the hero really appealing and heroic? Is the villain really evil? Is there a powerful and credible conflict? Are the characters great, real, alive, right and consistent?
Strengthening these big elements might result in major restructuring of the screenplay. Only after they have been addressed is the time right to work down to issues such as spelling and grammar. Take a break for a few days at any point if necessary to get a fresh perspective.
This rather strange term comes from William Goldman's books. It means getting together with another writer and discussing your script and coming up with ideas to solve problems you are having.
It's essential to discover whether or not your story, characters, dramatic conflict, is working. A screenplay is both a sales document for the film and the blueprint, a set of instructions to the director and the actors. An outside viewpoint is essential if you want to know if your script is going to be successful.
Script Editors 
A good script editor is invaluable. Professional editors and coverage experts can be located by searching the web and through ads in screenwriting magazines, however the cost can be prohibitive for independent filmmakers.
Another option is to enlist friends that are avid novel readers and particularly anal-retentive at spelling and grammar. Even the most successful professional writers value the feedback of their trusted friends. If the writing is interesting enough, friends will feel compensated by just getting to be the first to read the new work and share in the creative process.