Breathing exercises are a great way to help alleviate stress. There is a reason for the common term that is used to calm someone down, "Take a deep breath."
For a great breathing exercise you can do anywhere,
Inhale deeply through your nose to fill your lungs, puffing out your chest (pushing in your stomach). Hold it for 5 seconds. Then exhale deeply out your mouth, using your stomach to push the air out of your lungs (puffing out your stomach). Hold for 5 seconds. Repeat for a minute or so.
Fit's Tip: To really see your chest and stomach working, lie on the floor.