I've been experimenting with lots of different things. By far the most useful thing I've found is to take a running break for three to ten miles in the middle of the workday to break it in half. I have a standing desk and a sitting desk, and I've made a habit of standing or walking around when I have something to spend a long time reading. I tried sitting on an exercise ball at my desk and found it too distracting, but I found having a small exercise ball under the desk to kick around while working still lets me focus. And I've paid more attention to when I start losing focus while sitting and then to hop up and spend at least a few minutes walking around.