Get happy feet.
Relieve leg strain by using a comfy footrest under your desk (a couple of phone books works too); you'll have more energy at the end of the day.
Leave your desk for lunch.
At least once a week, drag yourself out for lunch. Skip the fast food and go to the nicest place you can afford. Or bring a sandwich and eat it outside. Having a bright spot in the middle of the workweek helps you make it to Friday.
Indulge in a small luxury.
I keep gourmet flavored tea bags at my desk. At $5 a box, it's not the cheapest brew around, but it's well worth the kick I get when I smell cinnamon and orange wafting through my office.