This is such a common question that I figured I'd be wise answer it!
There are two types of solutions - solutions that can be applied while at work and solutions that can be applied outside of work that will effect stress levels while at work. I will focus on solutions while at work first.
The key thing to realize is that you are responsible for most of your physical functioning. Stress requires a shift in your breathing to become stress, and a shift in your breathing relieves stress. Can you take a big sigh of relief right now?
If you are stressed, it is almost certain that you can sigh in relief. Just do it. You can be an actor if it doesn't feel "natural." Now that you've done it, I'm sure you feel at least a little better.
People can also benefit from the opposite approach - to tense all muscles and then let them go. Again, direct responsibility for physical functioning. A person can tense a muscle, and a person can relax a muscle. By tensing, a person directly demonstrates the ability to relax.
Once a person knows that relaxation is available at will, they are less prone to identify situations as "stressful" and can work with the subtleties that created the stress in the first place and harness all the abilities to make things better.
The breathing is crucial. It is a rare person indeed who can be stressed and breathe deeply at the same time. See if you can attend to breathing deeper the next time you discover yourself stressed. It works for everyone to improve the situation - and works even better to PREVENT stress.