Breath. Simplest meditation technique. Just focusing on my breath then switching my focus to the task, then switching back to breath. Also, if I really need to focus intensely, I slow my breath down. Similar to taking deep slow breaths but without a sigh through the mouth.
A workout followed by a break. It makes it easier to transition to a work session after you work out your body. I usually do biking or some other cardio based physical exercise. Preparation is key to productivity.