Problem solving - when really stuck try...

• A cup of tea/coffee
• Social chat with a colleague
• Walk in the *fresh air
• Visit a huge building with great architecture/decor

Most of those points are really obvious, but all too easy to forget, when engrossed in a tough Mathematical problem.

*Without realising it you might be really need the fresh air!
( If you work in a Science building and have your own small office, then sitting in there all day with the door closed, will reduce the air quality. )

Visiting the Science Museum in London, a local historic building, or a local Cathedral usually works for me also.

( The Cathedral is not my way of seeking 'something divine', but they are usually architecturally interesting and generally not full of hustle and bustle )

For some, a Distraction can also work, and there is a Psychological Science Article summary here giving some details.

The full 2008 research paper by Chen-Bo Zhong, Dijksterhuis, and Galinsky can be found at utoronto.ca here.

Another option, which sometimes works, is to pick a marginally related problem, and plan to come back to the original in a few days time.