When I was studying martial arts, we did some stretches that result in immediate increases in flexibility.
Stretch #1: Butterfly Stretch
part 1 Do a normal butterfly stretch
part 2. Have your partner put most or all of his weight on your knees
Stretch #2: Touch your toes stretch
part 1 Sit down, stick your legs out and try to touch your toes.
part 2. Have your partner put most or all of his weight on your shoulderblades.
While very painful, they can really increase your flexibility quickly or put you ion traction if your partner is a dumbass...