I've had an IN box for as long as I can remember. But it's only comparatively recently (a year or so) that I've had a formal OUT box. Here's what I've discovered about using it.
It's not actually a box or tray, like my physical IN box. Rather, it's more of a "landing zone" on top of the printer stand - a designated bit of clear space which, in my mind, carries the label "OUT".
When something needs to go out of my office, I put it there. Bowl from this morning's cereal? OUT box. Completed expense reports that need to be mailed? OUT box. Mail that needs my wife's attention instead of mine? OUT box.
In the same way that a pad of paper lets me capture thoughts without acting on them immediately, the Out Box lets me stage items that need to be moved somewhere, without interrupting my flow of work. I can let go mentally of the thought that this item needs to be put away. When I do want to take a break and stretch, all these "somewhere else" items are in one place so it's easy to scoop them up and walk them to where they need to be.
What I DON'T put in the OUT box are things "to be filed". I've learned that that doesn't work. It grows without bound, and then OUT is useless because it's cluttered. I file most things immediately, though I'm ashamed to say there's still a bit of a TO FILE pile that needs attention. (It's on my "IN OFFICE" action list to clean that up.)
So, the two habits I'm working on right now have to do with comings and goings: (1) when I enter my office, I need to toss any notes I've taken into my In Box for processing; and (2) when I leave my office, I need to check the Out Box to see what can be delivered elsewhere.
Simple things, but they help.