DO use static stretching to maintain flexibility, but do it after your workout, not before. Even doing a few static stretches at the end of a single workout will help with next-day muscle soreness so you won't be moving like a corpse.
DO stretch tight muscles when training a favorite or strong body part. For instance, if your chest is strong and your calves are tight—a common scenario—stretch your calves and increase the frequency of stretching without making it a boring chore.
DO use traction when stretching to increase range of motion and reduce compression or impingement of a joint. This can be done in the gym by pulling on a resistance band attached to an immovable object like a power cage or chin-up bar. Either grab on to the band with your hand for various upper-body stretches, or hook it on to your foot or ankle for a number of lower-body options.
DO control which area of the muscle is being stretched. To stretch the hamstrings, for example, you target the muscle belly when bending the knee, rounding the back, or plantar-flexing the ankle (i.e., pointing the foot away). If you lock your knee, keep your back straight, or dorsi-flex the ankle (i.e., flex the foot toward the shin), the target instead is the fascia, the sheath that covers the muscle.
DO scan your body for tight muscles, then attack the target area by stretching. Always stretch tight muscles first as they can inhibit your ability to do full-range exercises. (Note: This is a case when dynamic stretching is done before your actual training.) During your warm-up, use general movement of all body parts to scan for tightness. Once found, use the appropriate stretching techniques to release it.
DO favor closed-chain over open-chain stretches. Most people stretch their hamstrings by throwing their heel on a bench and reaching forward to their toes, which is an open-chain stretch. Research shows closed-chain stretching results in a 5-degree increase in flexibility. Any form of stretching that exerts pressure on the soles of the feet or the palms of the hands (which closes the chain) will produce strong reflex extension, and greater range. Toe-touching stretches done in standing versus seated positions are actually different procedures to your nervous system.
DO use gentle motion for rehabilitation, but don't push the end range. For instance, the popular "mad cat" and "camel" stretches that you see people do on their hands and knees are useful for neural flossing of the spine (by getting nerves to move, they can create their own space). If you experience back pain, 5-6 cycles of these stretches prior to training may help.
DO stretch surrounding muscles to liberate greater range of motion (ROM). For instance, the iliotibial (IT) band is a dense, fibrous band of connective tissue that runs along the outside of your thighs and is very resistant to stretch. To really get at this tissue, you need to address the muscles on either side of the IT band, such as the quadriceps and hamstrings. Rolling on a foam roller can help.
DON'T hold an intense stretch for longer than 15 seconds because of muscle hypoxia. Lack of oxygen to the muscles develops under a high degree of force/tension and can increase the development of connective tissue, which decreases strength and may actually promote inflexibility. It's better to use multiple angles for a short duration with static stretching rather than holding one angle for a long period of time. The rule is that the more intensive the stretching, the shorter its application.
DON'T skip strength-training exercises that promote passive stretching. These movements will result in an increase in flexibility, assuming you train using a full range of motion. Here's a list of the best choices, by body part:
DON'T stretch first thing in the morning, especially if you have a low back injury. Wait at least one hour after awakening. While you sleep, your spine swells with fluid, and the risk of injury is heightened if you stretch right after you wake up.
DON'T hold your breath during a stretch, as this will tense your muscles. Instead, you need to relax by exhaling longer than inhaling.
DON'T believe the myth that weight training will make you inflexible! John Grimek, a weightlifter and world-champion bodybuilder in the 1930s and 1940s, would perform back flips and splits during his posing routines. Tom Platz, a world champion bodybuilder from the 1970s and 1980s, also displayed extraordinary flexibility, considering he had arguably the best-built legs in all of bodybuilding. Platz would perform full, deep squats in training, and he was notorious for being able to not only touch his toes, but kiss his knees! That's how flexible his hamstrings were, so don't believe the hype!
I hope you find this information appropriate and useful!