It's simple. I feel, it's over-rated.
We are creatures of habit. Break a habit deliberately and you are out of the box. If usually take a certain route to a certain destination, take a different one.
I just visited Ahmedabad over Diwali. Good enough to break the rut I got into staying at the hospital and trying to manage it somehow. I've come back with more thoughts, clearer thoughts and a different resolve.
Boxes are tough to see. So it's tough to be out of the box :). This awareness itself is good enough. Remind yourself time and again that boxes are tough to see. And that habits are boxes. Break those habits; you'll find yourself out of the box.