Do just one thing. By which I mean: if I have a task list, when I choose the next task to do, I do only that task. I don't check email, read HN, answer phones, or anything else until the task is done. After the task, I allow the email check, surf, etc. for a few minutes, before I begin on the next task, which, once again, I do to the exclusion of everything else. If you do this for a few days, you'll find it's a progressively easier habit to move in and out of the 'concentration zone.'