This is a good question. In my case, writers block usually occurs when I place a value on my writing prematurely. I type a paragraph or two, then proceed to read it and reread it, thinking that it is not good enough. I then become discouraged and stop writing all together.
To solve this problem, I remind myself about the great value of the editing process. I now know that judging the content will happen much later and that the public will never see the imperfect paragraph as it stands when I first write it. This gives me freedom to create without paralyzing fear.