For me, the best way to relax quickly is a combination of deep breathing and muscle tension-release. This shakes out the stiffness while providing the oxygen that turns energy into excitement. I can do this with tension-release exercises, or by dancing.
If I want to deepen my relaxation (either after tension-release, or before I get tense in the first place) I practice mindfulness meditation. With that, I never get stressed out in the first place - I stay relaxed and excited, or relaxed and calm, most all the time.
As a coach, I've found that everyone is different. Some people benefit from: exercise, meditation, yoga, music, or relaxing with a good book.