I often wish that more screenwriters would share their knowledge of what it is really like trying to write a script. After years of almost non-stop writing, I am putting my pixels where my mouth is, and offering a series of tips - if only to remind me what to do when I get stuck.
1. Treat the writing operation like an open-source coding project - 'release early and often'. In other words don't stew on the story for months and years, rather get something out quickly and try it out on people (almost any vertical body will do, i have discovered).
After all, people are your audience, and when it comes to stories, many people have pretty good instincts. If you find that two rather different people say almost the same thing (either Yes, this is wonderful, or Oh dear, this doesn't quite work for me), then those are important signs.
2. Regarding getting something done quickly, I have seen years wasted by writers and other idea people by grinding away hoping for perfection only to discover that the world has either moved on or the idea took a major wrong turn early on in the proceedings.
Getting early feedback can really help the wrong turn problem, and if you so inclined, a more constant dialogue can also really help. It worked for Billy Wilder, who found it was almost impossible to write on his own - he had to have a writing partner.
3) The Research Trap: I have also seen people waste years on research. However, since it is a good idea to stay connected with reality in as many ways as possible (even in a fantasy story), some balance between facts or real connections and actual textual production has to be struck.
There are some amazing writers like Freddie Forsyth, who researches his next for about two years then writes in a forty-day blitz, but he is a master, and most of us have to settle for something more mundane and mechanical.
I have found that, in general, keeping research down to (lots of) small packets of research (sometimes less then ten minutes each, which sounds unbelievable i know) is best for dramatic writing, unless it is a whole new area you don't know much about and you need to read one or more books.
I definitely spent weeks researching the The Big Steal, that we are now in the process of raising money for, but that was because I found that I didn't really know very much about the real state of banking - and I am not sure that many people still do, despite the disasters of the last five years. The situation is so much worse than I thought, particularly the whole shadowy world behind offshore banking (which is truly mind boggling).
Finally, a disclaimer: if you have any other kinds of blockages, please seek medical advice and please don't try drinking Draino.