Though not really part of the GTD systematic approach, the MoSCoW Rules, often applied to priorising in project management, can be useful in helping you decide what order to do the tasks on your lists.
“MoSCoW” in this context stands for “Must o Should Could o Won’t”
Firstly, what tasks MUST you do today? These should be at the top of your list and headed “MUST DO TODAY”. Draw a line under this list.
Secondly, what tasks SHOULD you do today? These appear next on your list and are headed “SHOULD DO TODAY”. You should only tackle these when all your MUST DO list has been completed. Draw a line under this list.
Thirdly, what tasks COULD you do today, but only if you have the time and you’ve done all the MUST do, and SHOULD do tasks? List these under the heading “COULD DO TODAY” and draw a line below the list.
Finally, what WON’T you do today? Writing tasks on this list helps you to confirm that they’re not as important as anything in the three categories above. Also, it reminds you that if you catch yourself doing one of these tasks, you should stop and go back the top of your lists and work down again.
Working through these lists top down should motivate you to get the MUSTs and SHOULDs done so that you can feel good about doing some of the COULDs but not feel bad about not doing any of the WON’Ts!