Merlin Mann of 43 Folders has a great list of ways to break through writer's block.
Write crap – Accept that your first draft will suck, and just go with it. Finish something.
Unplug the router – Metafilter and Boing Boing aren't helping you right now. Turn off the Interweb and close every application you don't need. Consider creating a new user account on your computer with none of your familiar apps or configurations.
Write the middle – Stop whining over a perfect lead, and write the next part or the part after that. Write your favorite part. Write the cover letter or email you'll send when it's done.