Well you already got lots of responses, so I'm sort of repeating what other people have said...but.....
I get terrible writer's block. I think it's an intense case of performance anxiety. I just get scared I can't do it.
I don't have any great answers, but I can share with you some of my thoughts about it - things I'm thinking to help myself get through it. Something Stephen King wrote stuck with me - he says that he writes his first drafts as quickly as possible, so the voices of doubt don't catch up to him.
Also, someone recently wrote on Nathan's blog about how she writes on paper plates, rather than fine china - knowing everything can be thrown away, and the expectations are low. I think if there's anything we can do to lighten up, reduce pressure and expectations, and make it more fun, that can help.
Also, one thing that's working really well for me is having deadlines. I'm in school, and I can't afford to be blocked. I just write and stop worrying about it. I don't know yet if there's a way to recreate that in my own writing, but if there is, I'll use it. :)
A couple of books have been helpful to me around being blocked - The Artist's Way, If you want to write by Ueland, and a new one, If you can talk, you can write. I suppose there are probably books written expressly for being blocked, one of these days I might do some browsing at Amazon. :)
Finally, this is a really weird trick, but I am completely unblocked in blogs. So, sometimes, I'll start typing in the little box just to get myself started. I was thinking of starting a private blog and typing there, just because somehow it frees me up.
And really finally this time, (sorry this is a long message) just knowing I can only do the best I can, it doesn't have to be perfect. And let it sit for abit too.
Hope some of this is helpful! It's good for me to write about it, so it was helpful to me. :)
All the best to you!