Your body's reference point while standing is called anatomical position; where all the joints are vertically stacked upon each other from the top of your head, through your core, to the bottom of your feet. It's important to maintain this upright alignment to allow gravity to flow through your body and move through specific bony landmarks; thereby, decreasing unnecessary muscle tension and stress on your body. Unfortunately most of us don't have "perfect" posture and over a period of time muscle imbalances take us away from this vertical alignment. The 10,000 steps and 20,000 breathes you take on a daily basis are restricted and force other parts of our body to compensate to accomplish "simple" movements. Stretching can be an important part of your solution to increase flexibility and bring harmony to your body.

Stretching to the Rescue:

Before we get started it's important to understand that you shouldn't stretch out all muscles the same. The agonist muscle (e.g. biceps) will become short and its antagonist muscle (e.g. triceps) will become long and a muscle that's chronically shortened will be weak when it attempts to lengthen and a muscle that's chronically lengthened will be weak when it attempts to shorten. In many instances it's impossible to know if the short or lengthened muscle is the primary problem. The most practical solution for "a general flexibility routine" encompasses stretching the tight "agonist" muscle while activating the lengthened "antagonist" muscle or in other words contract the opposite muscle group you're stretching (e.g. stretch biceps/contract triceps); you're addressing both sides of the joint in a functional manner.

Types of Stretching:

In general terms, and to avoid getting caught up in the stretching controversy, there are two types of stretching: static (without motion) and dynamic (with motion). Yes, there are dozens of different stretching names, but they are mostly derivative of the basic ones we'll cover. As per usual it's up to YOU to decide what's needed for your lifestyle.

Without motion:

Static stretching consists of stretching a muscle as far as possible without pain and then holding that position for 20-30 seconds. This form of relaxed stretching is used to increase flexibility over a long period of time (as in months, not the hold of the stretch), during cool downs to prevent blood pooling and return the muscle to it's resting position, plus as a measure to relieve spasms and prevent adhesions in muscles that are healing after an injury. The term passive stretching is very similar and the main difference is the use of external force, either a stretching buddy or an exercise apparatus, brings the joint through its range of motion.

Proprioceptive Neuromuscular Facilitation (PNF) is a wide assortment of rehabilitative movements used to re-educate a muscle; however, we'll only touch upon the stretching aspect. The Contract-Relax technique take advantage of the post isometric relaxation in a muscle to increase its range of motion; for those interested it's occurs when the Golgi Tendons Organs/GTO sense tension overload and relax a muscle to prevent injury (e.g. a common ankle sprain).

After assuming an initial passive stretch, the muscle being stretched is isometrically (muscle force exerted, but no movement occurs) contracted for 5-10 seconds, after which the muscle is briefly relaxed for 2-3 seconds (GTO causes relaxation), and then immediately subjected to another passive stretch which stretches the muscle even further than the initial passive stretch. This final passive stretch is held for 10-15 seconds. The whole PNF process is to "trick" the muscle spindle fibers that contract a muscle when it's stretched too far, too fast. The method can also include contracting the antagonist muscle before going deeper into the stretch. For example, stretch your hamstring, isometric contraction of hamstring, and an isometric contraction of the quadriceps as you go deeper into the hamstring stretch.

With motion:

Dynamic stretching consists of controlled leg and arm swings that take you to the limits of your range of motion and not beyond as in ballistic stretching. An example of dynamic stretching would be slow, controlled leg swings, arm swings, or torso twists as part of a general warm-up for an activity. Your proprioception is allowed to keep track of these controlled movements by providing feedback to the central nervous system (CNS).

Ballistic stretching uses momentum rather than muscular control to increase ROM and allows for bouncing. This form of stretching uses the momentum of a moving body or a limb in an attempt to force it beyond its normal range of motion and is obviously used with discretion, because it can cause microscopic tears that tighten the muscle even further. Your proprioception loses track of such "uncontrolled" movements and there is no feedback to the CNS to alter the outcome of the stretch.

Important note:

There is a situation where "neural tightness" can make a muscle feel tight and the loss of range of motion is from an overactive nervous system (you are stressed out!) and not structural limitations. For instance, it's common for people with an anterior tilted pelvic girdle, and increased curve in the lower back, to have the sensation of tight hamstring even though the muscle is already in a lengthened state; so, you'd want to back off stretching the hamstrings and build core strength to stabilize the lumbo-pelvic-hip complex (your core) to address to the problem.

Conclusion:

How often to stretch is up to you and totally dependent on why you're stretching. As a very general rule of thumb it's beneficial to stretch out daily, but even 3 times a week will maintain flexibility. Lastly, remember to make stretching FUN or you won't do it; so, listen to music, stretch with a friend, and/or join a class with other like-minded people.