The Fall 1975 issue of Paris Review featured an interview with novelist John Steinbeck. He won the Pulitzer Prize and was a Nobel laureate, and The Grapes of Wrath and Of Mice and Men are still required reading in many English and literature classes.
Here are six tips from that interview (culled by the excellent Brain Pickings blog), with a few additional comments by me. First:
"Abandon the idea that you are ever going to finish. Lose track of the 400 pages and write just one page for each day, it helps. Then when it gets finished, you are always surprised."
The idea of filling 200 or 400 blank pages (or 120, for screenplay writers) is so daunting to many people that they never do write page one. In addition to focusing on one page at a time, as Steinbeck suggests, remember that you don't have to start at the beginning. If there's a scene somewhere else in the story you feel more ready to write, do it. Sometimes I'll write in a patchwork like that and in the rewrite make sure it all fits together.
(Writing advice from the classic and modern great writers is what you'll find in my book, Your Creative Writing Masterclass, published by Nicholas Brealey and available from your favorite bookseller.)