Stretching is useful for both injury prevention and injury treatment. For the purposes of this discussion I will concentrate on prevention. If done properly, stretching increases flexibility and this directly translates into reduced risk of injury. The reason is that a muscle/tendon group with a greater range of motion passively, will be less likely to experience tears when used actively. Stretching is also thought to improve recovery and may enhance athletic performance. The latter has not been fully agreed upon in the medical literature, but improved biomechanical efficiency has been suggested as an explanation. Additionally, increased flexibility of the neck, shoulders and upper back may improve respiratory function.
How to Stretch. There are three methods of stretching: static, ballistic, and proprioceptive neuromuscular facilitation (PNF). Static is the method recommended for the majority of athletes since it is the least likely to cause injury. Ballistic (bouncing) and PNF stretching are probably best reserved for a select few who are experienced with their use. To get the most benefit from your static stretching routine while minimizing injury, stretching should be done after warm-up exercises. The increased blood flow to the muscles aids in the flexibility gains from stretching and is an important component for injury prevention. Static stretching is done by slowly moving a joint towards it's end-range of motion. A gentle "pulling" sensation should be felt in the desired muscle. This position is then held for 15 - 20 seconds. Do not stretch to the point of pain and do not bounce since this may cause injury to the muscle. Within a session, each subsequent stretch of a particular muscle group seems to give progressively more flexibility. A set of 3 to 5 stretches is probably sufficient to get the maximum out of the routine. Alternate between agonist and antagonist muscle groups (eg. quadriceps and hamstrings), and alternate sides. It is also a good idea to start with the neck and progress down to the feet. This enables you to take advantage of gains in flexibility from the previously stretched muscle groups. Stretching should also be done after the workout. The post-workout stretch is thought to aid in recovery. Cold packs can be applied to sore areas in those of you who are recovering from injuries.
Why am I so tight? There is considerable variation in baseline flexibility between individuals. There may also be variation within a given individual (eg. flexible shoulders but inflexible hips, or flexible right hamstring, but tight, inflexible left hamstring). Genetics, injuries, and abnormal biomechanics all play a role in these differences. One shouldn't try to make big gains in flexibility in a short period of time. Stretching should be done gradually over a long period of time and then maintained to prevent slipping back towards inflexibility. Some people will enthusiastically embark on a stretching program, but then quit two weeks later because they haven't seen any benefit. Be patient and consistent. It takes a long time.
Relax It is very important to relax during the stretching routine. It should not be a rushed event. Don't think about your job and don't look at others working out. The "I've got to hurry up and do this so I can go" attitude is counterproductive. This is a time to slow your breathing and to free your mind. Some athletes will employ mental imagery while stretching. In this relaxed state, the athlete visualizes proper form in preparation for training or competition.
Final words If you have any back, neck, bone or joint problems consult your doctor before beginning a stretching program. No stretching routine should be painful. Pain indicates either incorrect technique or a medical problem. If in doubt, ask a qualified health professional. The following are some examples of good stretches.