Stretching helps muscles to relax and increase their range of motion. Also, stretching can lengthen the muscles and prevent them from becoming bulky. It is best to warm up for at least 10 minutes before attempting to stretch. Ideally you should integrate stretching into your muscle strengthening work at least three times a week. Even more if you are very tight.
Proper technique is important.
Imitate the cat who moves slowly and gently, never jerking or forcing its body into position. When the muscle becomes tense, hold the stretch but be careful not to bounce since this may make it tighter or even cause injury. As you relax into the stretch, you may find the amount of stretch increasing. A minimum of 20 seconds is necessary for any stretch. Longer is okay as long as you continue to relax into the stretch.