PNF stretching which is also known as proprioceptive neuromuscular facilitation stretching, is a set of stretching techniques that can increase both active and passive range of motion and provide real gains in flexibility in a really short time. Treating your body almost like a bow you pull to your maximum range while resisting, hold it long enough (usually 15 seconds) for the muscles and tendons to tire and the appropriate muscle fiber relaxation response to kick in and then you apply pressure again, stretching the muscles even more. To find out more about flexibility and muscle strength check out Stretching for strength and Flexibility.