These are best if you can do it with a partner to push you farther, but you can do them yourself as well. As mentioned before, you should hold all the stretches for 15-20 seconds, don't bounce. Also breath out while you're stretching and massage the muscles you're trying to stretch. All exercises should be done without shoes on.
1) Sit on the ground with your legs straight in front of you. Cross one leg over your other at the knee, then reach with both hands to the extended foot. Reverse your legs and do the same, then extend both legs and do it again.
2) Sit bow-legged so that the bottoms of your feet are touching and your toes are pointing away from you. Grip your toes with both hands and pull them in as far as you can, then use your elbows to push your knees towards the ground. Also lean forward as far as you can. This one really helps to have a partner.
3) While standing, bend all the way over with your legs straight and touch your toes, then bend as far back as you can.
4) ***PARTNER REQUIRED*** Standing about 1.5 feet away facing a partner of similar or greater height, have them squat down, then put one leg on their shoulder. Have them slowly stand up while you keep both legs straight. Switch legs and repeat.