What is it?
Static stretching is the most commonly known type of stretching. This is where you take a muscle to its full extension and hold it for a prolonged period of time, gradually increasing the range of motion when possible.
How it works
The body has a safety mechanism known as the stretch reflex. The stretch reflex is designed to protect muscles from being torn by their antagonist over extending them. When the muscle spindle senses that the muscle has stretched "far enough", it causes the muscle fibers to automatically start contracting, to prevent any further extension of the muscle. When the stretch is held safely for a long enough period of time (usually 20-30 seconds), the Golgi Tendon Organ senses that the stretch is a safe one, and begins overriding the muscle spindle. This ceases the auto-contraction, and allows the muscle to begin extending into a new range of motion.
Hold the stretch for a bare minimum of 30 seconds. You need to allow the GTO time to override the muscle spindle. Usually you can feel it, as the tension decreases or the muscle is allowed to increase its range.
Multiple reps is good.
Multiple times per day doesn't hurt.
Save yourself time and only use static stretching for muscles that are chronically tight. These muscles will be different for every person, depending on your personal postural deviations... but some of the most common ones are: Calves, Hip Flexors, Lats, Adductors, Piriformis.