Start your day by writing down the things you're already committed to doing such as meetings, time sensitive phone calls, appointments, etc. (the stuff David Allen calls the hard landscape) on an index card. Now, add 1-3 other things you want to get accomplished today. If you can't fit what you need/want to do on one side of an index card you're probably biting off more than you can chew.*

* A normal sized index card -- not one of those 5x7 monstrosities.