The jury is still out on whether stretching prevents injury, but there is no denying that it improves flexibility. At the very least, the flexibility that comes from regular stretching will help you resist the shortening of the tendons that comes with age, and help you maintain good posture. And for you athletes, stretching improves the ability to move easily in all directions, which makes you more efficient.
For martial arts, running or cycling, try this move to stretch the iliopsoas muscle in the front of the hip: start in a standing position and move to a lunge. Position yourself so that the front knee is directly over the front ankle, forming a 90-degree angle. Stretch your other leg back and rest your knee on the floor, with the foot of the back leg straight out or with the toes curled under. Keep both knees in the same position, and gradually sink the hip of the extended leg toward the floor. You should feel an easy stretch in the front of the hip. Hold for 30 seconds. Repeat with the other leg.
Whatever stretches you add to your sport specific workout, keep these principles in mind:

• Never stretch to the point of pain. When you feel a stretch in a muscle, stop and hold it at that point.
• Stretching is not a competitive sport and trying to stretch as far as you can--or as far as your training partner can--will only leave you with a pulled muscle.
• Remember to breathe normally. Holding your breath won't help you reach any further.