The longer (and slightly tangential) answer: Get out of the house. Leave the office. Get a change of scenery. Head down to the beach. Walk to the park. Some people like to say that our outside reality is a reflection of our inside reality. So, if your stuck on a project (can’t think of what to do next), then it’s possible that your workspace may also be ‘messy.’ By cleaning up your workspace, it may ‘clear up space’ in your head to allow you to think of what to do next in the project.
Back to the shorter answer: exercise. This is similar to the longer answer in that having your physical workspace (untidy) may be having an effect on your mental workspace. Let’s say someone is stuck on a project. They can’t quite connect the dots on something. They try and try and try by staring at the project materials to consider what the next avenue may be. Essentially, they’re stuck and can’t see the forest for the trees. By going and exercising, this person could shake-up their physical environment, which would have a cascading effect on their mental environment. There’s more to the science than I’m willing to get into this morning, but the next time you’re feeling stuck on a project, consider going for a walk, a run, a swim, or some other form of exercise.