Key for me is to get up and move about for a few minutes.
I use the Pomodoro technique (though I have doubled my times to 50min work / 10 min break). In my 10 min break I'll usually do something like making a round of tea, walking over to the window and looking out at something (a tree, the cathedral or just the sky).
Sometimes I'll be able to go for a short walk or sit on the toilet, it's amazing how many break through's come to me when I'm sat on the toilet - I think it's just the break and that I'm not trying to solve any problems so my mind is free to solve things its own way without me consciously trying to force the issue.
When I work from home I like to do some exercise; 100 push up challenge, do some chin ups or 10 minutes or so of yoga or tai chi. Harder to do those at work, but if you can find an unused meeting room it's pretty easy to open a kindle and read a few short pages from a book.