The "box" is the normal way of doing things and looking at things. It is the assumptions that almost everyone involved is making. The best way to start thinking out of the box then, is to identify and challenge all the assumptions that make up thinking inside the box.
One of the major liquor brands was faltering years ago, and they couldn't seem to boost their sales. Promotions, lowering the price, getting better shelf placement - these were the "in the box" solutions. Then someone challenged the assumptions, by asking "What if we stopped the promotions and just raised the price?"
The price was raised as an experiment, and sales soon doubled. As it turns out, some types of liquor are bought quite often as gifts. Buyers don't want to buy the most expensive one, but they also don't want to seem cheap, so they won't buy products that don't cost enough. Now imagine what happens to your profit margins when you raise the price and double the sales. That's the power of thinking outside of the box.
Ways To Get Outside The Box
Challenging assumptions is a powerful creative problem solving technique. The difficult part is to identify the assumptions. If you are designing a new motorcycle, write down assumptions like "speed matters," "it has to run on gas" and "it needs two wheels," not because you expect to prove these wrong, but because challenging these can lead to creative possibilities. Maybe the time has come for an electric three-wheeled motorcycle.
Another way to get to creative solutions is to "assume the absurd." This is either fun or annoying, depending on how open-minded you can be. All you do is start making absurd assumptions, then finding ways to make sense of them. The easiest way to do it is by asking "what if."
What if a carpet cleaning business was better off with half as many customers? It seems absurd, but work with it. Hmm...less stressful, perhaps. More profitable if each customer was worth three times as much. Is that possible? Commercial jobs that involve large easy-to-clean spaces (theaters, offices, convention halls) make more money in a day than houses, with fewer headaches. Focusing on getting those accounts could be the most profitable way to go - not so absurd.
Another way to more innovative ideas is to literally do your thinking out of the box. Get out of the house or the office. Look around at how others are doing things. On busses in Ecuador, salesmen put a product into everyones hands and let them hold it while they do a sales pitch. Then you have to give back "your" product or pay for it. It is very effective. How could you use the principle in your business?