As a kid I used to space out a lot. I could do it at the drop of a hat, in a crowded room, or even in the middle of eating. Although not as often, I still space out today.
|Just honk if you're behind me and the light turns green.|
|And if you don't shower at all you probably smell like a hobo.|
|Unless you've got, like, a super rockin' bod.|
It's both easier and harder than it sounds. I imagine it's a bit like meditation, except instead of letting go and emptying your thoughts to achieve nirvana or the like you're merely providing your brain an opportunity.
Basically, you want to unplug from everything. No Internet, no TV, no phone, no people, not even a book. You want to make your brain go from focusing on the outside to turning toward the inside. You want it give it an unchanging environment so that it doesn't have to use all of its power processing what's going on. It's why so many authors having rooms dedicated to writing (Neil Gaiman's writing gazebo being a personal favorite of mine.)
Granted, sometimes if you find your brain spacing off about the wrong things (the car, the house, the kids, what's for dinner, work, etc etc) you have to give your brain a little nudge in the right direction. Think of the last scene you wrote, or think of a different scene, or a scene in the future. Play with some ideas, your characters, go through some scenes in your head you don't plan to have in your book.
If you need to, open up to the last page of what you've written, but don't actually write. Sometimes I find myself doing that. I don't stress that I'm not actually writing though, because I'm working it through in my head.
If anyone saw me staring glassy-eyed into the computer screen and asked what I was doing, however, I would tell them "Writing" because, in my mind, that's technically what I'm doing.