Decide the night before what task is most important and let it be the first thing you do when you arrive at work. During the first hour upon your arrival will be when you will have the fewest distractions and the highest level of energy. Try not to spend more than an hour and a half on that particular task. Come back to it if you need to. Remember that the goal is not to complete as many tasks as possible nor to put in the work hours but rather to produce valuable results.