While sitting down, hold your palms out in front of you and turn them face up. Swing your arms out to your sides; roll your shoulders back so your shoulder blades are pressed together, then drop them as you continue squeezing. Pull your head back so you feel like you're giving yourself a double chin, and hold this position for 15 seconds.
You should be able to feel your muscles contracting, and when you release, you'll feel some of the tension fall away. If you find yourself fighting tension in other muscle groups, take some time to stretch everything out - these exercises can help.