Head down, butt in chair, hands on keyboard. Seriously. I power through it and keep writing. The problem with getting writer's block is that it usually means I've either taken a wrong turn somewhere, or I'm too stuck in trying to write the "perfect" book. So, I take a day or two break from the manuscript, then get back to it. It's never easy, of course, but I often find that forcing myself to write through it helps me see what was missing.