Get organized -- use a daily planner, prioritize tasks.
Learn to set limits. Don't agree to unnecessary, stressful obligations.
Be physically active and eat a healthy diet.
Get eight hours of sleep each night.
Don't take illegal drugs and limit your intake of alcohol and caffeine.
Stop smoking. Take regular relaxation breaks instead of smoking breaks. You can follow a similar routine -- leave the office, get fresh air, socialize and take slow, deep breaths.
Try some relaxation techniques, such as self-hypnosis or guided imagery, prayer, meditation, yoga -- whatever works for you.