The best advice I ever read about writing was in a Paris Review article - it was an interview with Ray Bradbury (it's a great interview, btw, and free to read online). In that interview, Bradbury wasn't replying to a question about writer's block, but his answer surely applies to it. Given that I can't possibly do better than him, I'll just post his answer here:
"Action is hope. At the end of each day, when you’ve done your work, you lie there and think, Well, I’ll be damned, I did this today. It doesn’t matter how good it is, or how bad—you did it. At the end of the week you’ll have a certain amount of accumulation. At the end of a year, you look back and say, I’ll be damned, it’s been a good year."
In other words, just write. Write your way through it. Other solutions include going for a walk or - for the Hemingway types - hitting the bottle.
I'm more of a fan of the Bradbury approach. Just keep writing. Even if it's terrible you can always rework it during the editing process.