There's little that's more frustrating and saddening to see than a writer hung up on their own ability to just write. The pressure to produce something, anything is so strong that, in the end, nothing comes out at all. The internal censors are too loud and too strident, and the pencil ends up meeting the page maybe once. It's doubly frustrating when you are barely able to eke out a couple of pages at a time, and then you know people who can sit down and hammer out thousands of words as if they were turning a tap.
It isn't hard for me to just sit down and write, partly because I've made a career out of doing it (albeit not writing fiction). But I'm close to a number of people who are not like that, and they are deeply frustrated by it. To them, I can only share the advice that I took to heart which, in time, helped get the tap opened up:
Free-write every day. This is the same advice that Peter Elbow gave his students through Writing Without Teachers: if you are in the habit of sitting down and writing freely — not diary entries, not letters to Mom, just writing — then you will become that much more accustomed to sitting down and writing something specific on demand.
Do not substitute anything else for free writing. In other words, don't write a LiveJournal post and call that your daily free-writing allotment (unless, of course, you're using LiveJournal as the repository for same or something like that). This part is critical, if you ask me — because if you don't practice free writing as its own animal, unhindered by anything else, you tend to think about writing exclusively as something where you have to produce something coherent instead of just producing. Which leads us to:
Do not worry about quality. This is the toughest hurdle to get over, but one way to get over it is to simply not show the results to anyone else. They're not meant to have an audience in the first place, no more so than anyone needs to watch you working out in order for you to get the benefits of the exercise.
The most important thing is to cultivate the right state of mind. Don't use free-writing time to produce something of "quality"; use it to learn how to put yourself in the state of mind that you will use later to produce something of quality.
In time you can indeed replace regular writing with free writing. I know this breaks rule #2, but that's kind of the point. Once you get the tap opened up and discover the state of mind you need to be in to write freely, the exercises required to get there may not be needed anymore. (Once you get to the other shore ... ) And then you can start thinking about what stories you want to tell, now that you've given yourself freedom of speech.