Some of the best advice I've ever come across regarding this is just to write anyway. Even when you don't feel like it, put words on paper. Even if it's gibberish. Just keep your pen (or keyboard) moving. Sometimes this seems pointless, and when it gets really frustrating, exercise generally does the trick for me. I've found that I often have to take pen and notebook with me when I'm on my elliptical machine, because that's sometimes where my best ideas happen.