Important Caveat:
This is the entire reason for the GTD system - to take action and move forward what is most important to you. The goal is not to organize - it is to take action! Many people get enamored with the GTD system and focus so much on the organizational aspects that they get fixated and don't act.
While it is great to refine your organizational system over time - once you get the first four steps done begin to live and work out of the system. All you need to do at that point is spend 15 to 30 minutes maintaining the inegrity of the system.
Taking Action:
David Allen indicates that while we might have to schedule some things - it is best to avoid scheduling every task. His rule of thumb for deciding what to take action on:
Leo Baubauta adds a great step to the weekly review process. He recommends that for each week you establish 3 "Big Rocks" or goals that are most important for you to accomplish that week. In fact, the ultimate goal is to focus on those 3 goals or "Big Rocks" first so you are actually moving forward what you care about most. This tends to build momentum towards achieving your longer term goals.
From here - your orientation will be to maintain a flow of action through each day of the week. You may have a day of a lot of meetings and on other days hae some gaps in your schedule. When you find the gaps focus on actions as follows:
Now go forth and forward your work and life!