They key is to compartmentalize your workweek, your workspace and your thoughts.
For example, I allow two days a week for meetings. These could be mandatory staff meetings, new business pitches, sales calls, anything that requires me to be in a room or conversation with someone, whether physically or virtually. Mondays and Fridays are my meeting days, and I’ve accepted that I won’t get much productive work accomplished on either of those days.
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Tuesday, Wednesday and Thursdays, however, are as interruption-free as possible. My assistant knows to play gatekeeper for me, and she doesn’t schedule anything on my calendar if at all possible.
Before I leave my office Monday evening, I clean my desk, throw out anything I can and file the rest. When I get into the office early Tuesday morning, I have a blank canvas, and a to-do list that I can start immediately, without any fear of interruption.