... here's one more trick that I just came across here and love the sound of, although I haven't tried it yet.
As a last resort, reboot your day. This is something I do not just with work, but any time a day is going so incredibly badly it just can’t be saved. Turn off the lights and take a quick nap. 20 minutes is best. When you get up, take a shower, make a small breakfast, and just go through your morning routine as if it were the first time. Try to forget anything prior even happened.
It's from a list of tips for computer programmers, but I reckon most of the tips on the list are applicable to other work as well. (But hey, dude, blue and red text on a black background? Not cool.)