I used to read Alan Lakein and Covey and they would stress the importance of planning. I would read about ordinary people who would start each day by planning.
I just didn't get it. It didn't work for me. I wasn't sure what to plan and what not to plan. And, of course, at work I never knew what was going to happen which might blow that day's plan out of the water.
Now I know how to, and do, plan. I plan during my weekly review. And I plan throughout the day when I scan my Next Actions and Projects lists.
This is the briliance of GTD at the level of what DA calls the runway and 10,000 feet. I can only plan successfully if I have put all of my commitments in writing. The ugly term that I use for writing is "externalizing," since I am putting my internal thoughts in some external form. I believe that my term is less ugly than DA's "distributed cognition" but in matters of taste who's to say?
The point is that it is very difficult to plan if one relies solely on "psychic RAM" (now there's a clever coinage!). One reason I never planned consistently and successfully pre-GTD is that I did not clearly define for myself what to put on the plan and what to exclude.
Now that I have all my commitments listed external to my mind I can engage in some productive planning. Planning means reviewing what I have already externalized as my commitments, reviewing what is internal only and needs to be externalized as my commitments, and making judgements about which commitments I want to focus on in the upcoming days. Planning requires a complete Next Actions list, Waiting For list, Projects list, and file system.