Proprioceptive neuromuscular Facilitation (PNF)
This is a very effective method of dramatically increasing flexibility, as well as being a little painful.
Silveraw mentioned it in his post.
it is performed by performing a Stretch, Relaxing, then following up with another Stretch.
Relaxing the Muscle between Stretches enable the following Stretch can be deeper.
With this method, often a partner perform that Stretch; increasing the force elicit a greater Stretch.
No partner, no problem.
Loaded Stretches
Loaded Stretches enable an individual to perform them without a partner.
Examples
1) Lats
Hanging from a Pull Up Bar.
2) Pecs
Performing the Stretch with Dumbbells below the chest.
3) Quads and Glutes
Sitting in the lowest position of a Squat with a Bar on your back.
4) Hamstrings and Lower Back
Perform a Deadlift Load Stretch by standing on an Elevated Box which does not allow the bar to touch the ground.
Contraindicated Stretching
Prolonging a Long Static Stretch prior in the Muscle in an Exercise is contraindicated.
Doing so, dampens Force Production; especially in Stretch Reflex Movements.
Antagonist Muscle Stretching
Stretching the Opposing Muscle (Antagonist Muscles) in an Exercise increase Force Production Agonist Muscles (Muscle being Trained in the Exercise).
Bench Press Exercise Example
Stretching the Lats prior to a Bench Press will increase Force Production (Power and Strength) in the Bench Press.
The reverse is also true...
Lat Training Exercise Example
Stretching the Pecs prior to performing a Lat Pulldown or Bent Over Row increase Force Production in these Movements.
Hypertrophy Training
Stretching between the Muscle in an Exercise after performing has been demonstrated to increase Muscle Mass.
Bodybuilding Coach was one of the first to use this method back in the 1990's.
Dr. Jake Wilson's (formerly Director of the University of Tampa Human Performance Lab) found Stretching Between Bodybuilding Exercise Sets produce an increase in Muscle Mass.
Strength/Bodybuilding Coach, Chris Thibauduea, has written about the effectiveness of this method.
These are thing to consider when implementing a Stretching Program.