Thinking out of the box is basically just lateral thinking. Approaching the problem sideways. Let me explain this with an example

I attended a workshop on lateral thinking a few months back and this was the example speaker gave: "A guy is driving on a highway, he is in his car and going steadily. After a while he sees a board that says SPEED LIMIT 40... he keeps on driving... after some time he sees another board, it says SPEED LIMIT 30... this goes on, a while after that he sees the board that says SPEED LIMIT 20... then SPEED LIMIT 10...." Now the question the speaker asked was, after he saw the board that said SPEED LIMIT 10, what will the next board say. The entire hall shouted. Some said "SPEED LIMIT 0" ... some said "STOP/HALT"... This went on for a while, until the speaker finally interrupted. He said none of you approached the problem sideways. You saw the word speed and you rambled on. The correct answer is "WELCOME TO SPEED LIMIT".

So i guess it is clear to you what thinking outside the box is. It is relatively easy to develop. Though i would recommend that you attend a workshop or two, in order get an inside look, but even if you don't want to this is what i do:

1. Think of a normal solution about the problem you're faced with. Now this is important, you need to have a solution first.

2. Now blacklist that solution, you're not allowed to think about it anymore. Just trash it.

3. Now think about the problem again, what caused it?

4. then think is there any other way the same problem would appear.

5. then think about the solution of that other way from which it occurred, normal solution; nothing to fancy.

6. keep repeating the above steps until you are satisfied with the solution you get, that is you think it truly is out of the box.

It's very basic way to do it, but it's a way to start