The best task management system I have used so far:
- Write down three to five most important to-dos in the morning*
- Do the hardest things first
- Say no to everything else (you can say no to)
- Check your progress on the list throughout the day
Discard the list at the end of the day. Start again the next morning.
*I usually use a post-it note or my journal. I have experimented with many productivity systems in the last 15 years. They are all too complex. Keep it simple.