I don't really see it as writer's block. For me, it's book block. I'm stuck on the characters or the plotting or the logistics. To get unstuck, I do an assortment of things--participate in physical activities that allow my mind to wander (driving, showering, walking), participate in mental activities that engage my conscious brain but allow my subconscious to wander (do jigsaw puzzles or play computer games), or I have a glass of wine. For some reason, one glass of wine really helps.
But I don't ever have "writer's block," where I can't think of something to write. It's always problem-oriented--"I must figure this tough part out."