Try and maintain a steady heart rate over a long period of time. Avoid cardio workouts that increase and decrease in intensity. You should be able to talk comfortably throughout the exercise. When weight training do more reps at a lighter weight over longer sets. Work the muscles where you hold stress.
People tend to carry their stress in their shoulders and upper back.
The result is tight muscles and poor posture. Strengthening and stretching these muscles can alleviate both of these problems. Stretch, stretch, stretch. Try a class such as Pilates or yoga that strengthens muscles by stretching them. Don't forget to breathe.