I've found that MOVEMENT is the key in our home when one of my boys gets fidgety or lacks focus. Since we homeschool, we have the flexibility to really limit seat-work, but when we need to get some done, I find that frequent breaks are so helpful.
Our brains receive much needed oxygen when we move, stretch and run around. Whenever my oldest son gets restless, I turn into 'drill-sargeant' mode and demand 10 push-ups. With glee, he drops the pencil and does 10 - bootcamp style. We proceed to race laps around the house, shake out the silly's and then sit down to tackle some more problems. We set a goal and then head out for a walk afterwards!