Some years ago I read few sentences that worth all GTD books written. It was in Russian and here is my interpretation in English:

There is a simple way to organize your time. It is called Progressive JPEG method. Each second you project is 100% ready although it can be 4% worked through only.

It depends on the time you have. A project can be either worked thoroughly till each pixel or can remain on the conceptual sketch stage.

Earlier you worked through all projects by 97%. You optimized all pictures, took into account each micron and polished all surfaces. Now you can enjoy the moment that nearly any task/problem can be solved by the time you outline it.

Progressive JPEG Method

In other words you spend your time only to get the right level of steak doneness while steak itself is ready right from the beginning.