Learned this one in my Eurhythmics class:
In a quiet room where you wont be disturbed, lie flat on the floor and close your eyes, becoming very self aware of your body. Now, do a "body check". Start with your head, your face, your jaw, making sure everything is 100% relaxed. Make sure you have no tension in your neck, or shoulders. Let every part of your body relax, until you feel as if you are floating. Take several slow deep breaths, each time making sure to leave your lungs feeling open, and think to your self that they are open. RELAX
Now that you're 100% relaxed, imagine you are somewhere you like (as a suggestion, lying on the beach is pretty easy to imagine since you're already lying down), and focus on that thought. Picture how your pace looks, how it sounds, how it smells, and how it feels. Relax, allow your "place" to take over your thoughts. Block all other thoughts and worries from your mind. If they come up, clear them away, and go back to your place. Don't fall asleep... Once you've had enough, slowly start to come back into the room, listening to the sounds, finally opening your eyes and slowly sitting up.
Listening (actively listening, not just "listening")to music is also a great way to let off stress. Orchestral music in particular is great to listen to for relaxation, and one of the best ways to get rid of stress that I can think of. Find a song that has soft, and active moments. Eric Whitacre has some really amazing choral/orchestral works that are nothing but pure joy to listen to. With out going into too much detail on how it works, I really recommend orchestral music over other types of music for this, since orchestral music (especially modern stuff) has FAR more harmonic complexity, which causes a very strong tension-release-tension-release relation ship in the music (doesn't really happen much in most popular stuff), that you can actually "feel", both emotionally and physically. Hearing this tension-release relationship is VERY therapeutic (kind of like watching waves in a way), and will 100% guaranteed leave you with a lot less stress.