I do most of my writing on a computer, so whenever I run into a brick wall, I just take out a pencil and a piece of paper and begin penning out my ideas. Then, I brainstorm until I just can't help but dive back onto my laptop and begin typing again.
But of course, there are those other days when my brain just feels like a lump of pudding. On days like that, I force myself to write at least a paragraph, and most of the time, that creates more ideas for the next paragraph and the next. Sometimes, you just have to push yourself through it.