It might sound funny and rather old fashioned, but using paper saves you time!
Simply write on a peper everything you need to do on that day (make sure you do not delegate this task to secretaries or other people, as it is your duty to sort your own things out).
When you have everything written, it is time you classify the "urgency" of the tasks as well as their "time to get done".
Start with what is fastest to get done and more urgent. Then move to tasks that take longer to get done.
Why postpone something that can be done in 5 minutes to do something that takes an hour? The more tasks you get rid of first, the more time you will have to worry about what takes longer to get done!
At the end of the day you will have solved more tasks :)