From Tony Schwartz at Harvard Business Review:

"Do the most important thing first in the morning, preferably without interruption, for 60 to 90 minutes, with a clear start and stop time."

I cannot agree more.  Tackling the more challenging tasks of the day, at least for me, requires a fresh mind.  I also find that getting my daily priorities organized the night before (just before leaving work) works well for me.  Not only does it help me stay on-task, but I sleep better knowing that I have a handle on what lies ahead.