One of the method could be pomodoro technique. You make a plan in the morning and list the task for a day in importance order. You get slots, could be 25 minutes work and 5 minutes break with a timer. When you have a work time you hocus ONLY on the task. If something is coming make a note that it is there but don't change your task. On your break make some easy task, check emails, quick chat. For a task get 4 times by 25 minutes, after get a longer break.
The idea is not about making time slots but about focusing on important task at the moment and only on this task. If someone approach you tell him you call back in whatever left ~18 minutes. The slot is short enough (25 minutes) that if you will not check your email, the world will not fall apart.
In addition to it when you open your browser do not open on twiter, facebook etc bookmarks. Switch of your messenger instant ring, at the most you will delay checking your emails by 25 minutes only.
During the day as you write down important, unplanned task you modify your list, but when you get to the end of your 25 pomodoro slot it can turn up that instant response is not so important any more.
The technique is not only about managing your time but at most about learning how to react to disturbances.