This is a very good question. When I do encounter writer's block, I initially try re-reading what I've written so far, but I read it not as an author, but as someone who just wants to read a new story. In doing so I tend to let the story carry me and as strange as it may sound, after I reach a certain point, the characters begin to speak to me once more. My mom once said that there will be times when your characters wont let you sleep, you'll be waking up at odd hours of the night with an urge to write...and as she predicted it has happened. I also listen to a lot of music, my stories have a soundtrack that drives not only the character, but gives the plot life and weight.