Get up out of your chair and walk briskly for five minutes; around the building, up and down the stairs, around the block - if it's not too large ...

Get some air into your lungs, move your legs and arms, stimulate your exhaust system, relax your eyes, take your mind off your work, smile at people ...

Finish up by bringing a glass of water back to your desk and getting on with your work as though nothing had happened.