Here's what I'm doing lately to cure myself of sitting down and going, "Oh man, what do I write?"/"I can't think of anything!" (I'm in the midst of writing a novel, so this may or may not apply to you)
Before I stop for the day, I write a few short notes within the ms about what I want to happen the next day, in general terms. At least then, I have something to start with the next day. There are a lot of variations to that, like stopping in the middle of a scene, or even in mid-sentence.
But, really you just need to sit down and start writing words. Maybe you just need a warm-up; try free-writing for fifteen minutes, writing whatever the hell you want - sometimes you may just need to free yourself from the piece you've been concentrating so hard on. I think learning to free-write has helped me enormously.
For the most part I think DISCIPLINE is the key (I'm still learning this, but doing better!); even if I wrote crap for the day, at least I wrote something, and that gives me confidence that I can write more the next day.