When you work on a problem, sometimes you become stuck in a rut.
Sounds familiar?
You sit with the same problem trying the same approach over and over again, hopefully with small variations (read: banging your forehead at the monitor from slightly different angles).

This is when you can be greatly helped by taking a step back and rethink your approach.

You can for instance change the scope of your focus for the current problem.

Perhaps you have been visualising the problem on a too low-level to see the full picture properly.
Or perhaps you have had a too wide approach and not quite gotten started at all because of the sheer size of it all. If so it is perhaps time to categorize and take a closer look at the different parts of the problem.

You can also change the technique you are using altogether:
"Ok, so I can´t break the product using boundary testing, perhaps if I use special characters instead that will reveal some new information for me". Or:
"I´ve been debugging this code from the end of the system and what comes out of it, lets take a look at the beginning and what input is sent into the system".

This is something James Bach calls "Focusing and De-focusing", in other words changing the way we are looking at the problem at hand.
He teaches this technique in his class for "Rapid software testing" and I can full heartedly recommend you take the class if you get the opportunity to.

This technique might sound easy enough, but the real trick lies in practising this often enough so that it becomes a habit and we don´t even have to think about it when we get stuck with a problem.

In the beginning I found myself (and still do at quite a lot of times) working on a problem like debugging code hours on end using the same technique (read: banging my head on the monitor) and suddenly realised I got nowhere and had to stop to change my approach, very oftenly this quickly lead to pleasant results.

Sometimes it isn´t even very important what change you do, just that you take a new approach and through that process get new ideas and information for the problem at hand.

This might not sound like much new under the sun, but you´d be suprised how often you find yourself stuck with a problem using the same approach over and over again, leading to frustration and draining of energy and attention.