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For knowledge workers, there is a lot of work to do defining our work before we can start actually doing our work. The 5 phases of workflow is the GTD model for capturing, defining and doing our work.
The habit of doing something as soon as it shows up if it will take less than 2 minutes. The idea is that this is roughly the point at which it will cost more to manage the item in your system than to just do it.
The practice of setting aside dedicated time (typically a couple of hours) to make sure your system is clean, current and complete. David Allen regularly cites this as the critical success factor for GTD.