I often completely leave the room. There are lots of options; pen and ink in hand I go to the cafe, sit at the kitchen table, out on my deck, anywhere I can still write. The key to remember is this is not a break from writing. You still have a job to do but you are using different muscles than when you sit in front of the computer.
OK, you have a plan and still the nagging cursor is flashing or now you're staring at a blank piece of paper. This usually means I haven't done enough mental writing. When I am in the midst of a project, I am always writing in my mind. Most writers do, that is why we carry notebooks with us.(Heaven forbid we miss a flash of inspiration or a great idea). But I am talking even more basic then that. For me mental writing happens everyday, even when I'd rather it didn't.
I have often been accused of spacing out or daydreaming. Usually, I am lost in a description, a visual image or even a dialogue with a character. Crazy? Maybe. I call it a writer's frame of reference. An artist is always storing visual images, a writer does too. However, we also collect words, feelings, descriptions...We take in the world around us in literary terms, we write it in our brain. Do we create perfect sentences? Of course not. When a non-writer looks at a chair, he sees a chair. When I look at a chair I see its rich wood grain, its worn seat and the high strong back. Get the idea? There is always something to write.
What is your cure for the flashing cursor?