To put the age-old wisdom of "work smarter, not harder" into practice, Startup Workout's Kevin Fleming suggests using concentrated "power hours" where you unitask on that one project.
Completely commit yourself to work on something for one hour. At the end of that hour, decide if you want to keep working, and if so, fully commit for another hour. By doing this, you are forcing yourself to remain focused. Once that hour is up, you can do whatever you want but during that hour you need to give 100% of your available energy to the task at hand.
We have previously suggested that you are likely to get get more done if you work in your "good hours", so applying the power hour technique to that could boost your productivity. If you are someone who takes a little time to get into the zone and fully concentrate your efforts into one task, the power hour can be a better tool.