How to manage To-Do List

I have been inviting the best system how to manage my To-Do list. Finally I decided to split my tasks into 3 categories:

• Next 24 Hours
  
• Next 7 Days
  
• Long Time
  

I use the MS Outlook (the obsolete version 2002 which came with my HTC P3300 mobile device) for my agenda. I can tag each task with the priority flag so I use High for "Next 24 Hours", Normal for "Next 7 Days", and Low for "Long Time". Then I made Outlook sort those task descending by priority. To manage my task I switch the priority flags manually.
And it works good for me.