Do something unrelated and don't do it with a furrowed brow. Try loud music or a long walk. Enjoy yourself. Remember, your writing will be there waiting for your return. That might be in five minutes, five hours or even five days. But it will be there.
It's not like someone is goose-stepping around you with a stopwatch. You are the boss, the manager, the under-manager, the shop-floor worker, the tea-maker, the dinner-lady/man, the cleaner, the secretary and the owner of the whole shooting match.