Cool your fingertips with a chilled can of soda, then place them on your eyelids. Using slight pressure gently roll your fingers outwards in a circular motion. This relaxes your eyes if you've been staring at your computer for a long time.
Lean back on your chair and stretch every muscle you can - down to your little toe muscles. Stretching may prevent back pain and muscle ache.
Buy a bunch of pretty flowers and arrange them in a vase. Flower arranging can be therapeutic even if it's only for five minutes. Once you're done, place them on your desk. They should cheer you up for at least a week.
Keep a pretty mug at your desk, and make it a point to have tea and cookies at a certain time every day.
Take five-minute breaks to zone out every alternate hour. Close your eyes and clear your mind for a little while. This helps increase your focus and productivity.
Put all work aside, pick up your phone and call a friend to catch up. Maybe set up an appointment to meet after work - that way you'll have something to look forward to at the end of the day. Try not to talk for more than five minutes though.
Had a horrible experience? Whine, and complain to a close coworker. It helps to vent. Just make sure no one's eavesdropping.
Staying in for lunch? Don't sit at your desk as usual. Take five minutes to place your food on a plate and head out to an empty meeting room. Eating in peace makes a difference. If you really have to, leave a note at your desk to say where you are.