On church youth group mission trips, I found that the days divided neatly into blocks of time. We would have breakfast, a devotional, work for an hour and half, have a break, work another hour and half, eat lunch, work another hour and half, have a break, clean up the site, and head back for showering, dinner, an evening devotion, and then downtime. We completed what we could in the time we had, took our breaks, cleaned up, and left, and each day had this monastic rhythm to it. The real enemy was time, which always wins; at some point, it's time to quit.
In reality, all our days are like this. We work in blocks of time and take breaks, and at some point we go home (or leave our home office). One important trick is maximizing those blocks of time to get the most possible done to the highest standards. Another vital trick is avoiding the interruptions that segment those blocks into smaller and smaller, and ultimately useless, blocks of time.
Avoiding interruptions during those blocks of time is especially difficult when we are surrounded by so many points of interruption: phones, cell phones, email, facebook, open office doors, dishwashers to unload, our fear of being rude to someone, and so on. With the light bulb and electricity, we can work long and comfortably into the night – at least, until fatigue saps us (and often ruins the next day). In contrast, Wendell Berry lives and works with simplicity, writing during the daylight and going to bed when it's dark, and other such practices that remind us of our basic limits as creatures.
There is no magic here, just like there's no magic to exercising or practicing one's religion--showing up is, after all, half the battle. Only through the drudgery of repeatedly showing up will such habits become natural to us. Habits take much longer to form than we admit, so it's no surprise that February is when people start dropping off from their new year's resolutions.
Whatever it takes, protect those blocks of time. Make sure that the interruptions that do occur are important and worthwhile, as some interruptions are. In the end, knowing your limits, when to work and when to leave, is essential.