If you start a day without a clear plan about how you're going to spend your attention, you'll end up wasting most of it. Your first priority should be to take a little undisturbed time each day to evaluate the various demands on your attention before they show up. Do your prioritizing whenever you typically think most clearly (most people do best in the morning, but I like to take five minutes before I go to bed to preview the upcoming day). Rank tasks in order of importance and write them down. That way, when you're being hounded for attention you'll have a visual cue to help you focus on the most significant task first, leaving less necessary items for later—or for never.