There are five certainties in the life of a graphic designer: death, taxes, and deadlines, deadlines, deadlines. It's a blessing and a curse really.
A part of me is very accomplishment oriented. I love working against a tough deadline. Even if I pull an all-nighter to beat the clock on a project, I'll get such a euphoric high once I finally put a design to bed, I won't feel the least bit tired.
Of course the deadlines never stop coming. Especially the most dreaded, "we need this yesterday". Tensions can build up. Designers can burn out... if they aren't careful. A few things help. I'll just start with my big three.
One thing that really helps me is taking a 10 minute walk. Seems counter-productive to take time off of a project but fresh air and a little exercise really refreshes the soul and sharpens my mind.
Music. 'Nuff said really. Especially for all-nighters, I kind of size up tasks within a project by cds. "I'll get the front of this brochure finished by the end of Sea Change and then get the insides laid out by the flip side of an Elvis". Lectures/podcasts are another favorite.
Good support = good eats. I have a great boss that knows sometimes the best support for a designer is to feed them. I think I hear my fellow designers all giving that last statement a big AMEN. Carbs can keep the creative motors running.