Legs behind the back
Preparation. Sit on the floor with straight and put one leg on the thigh of the other leg. Now bend the knee so that the leg resting on your thigh slowly is pushed towards your chest to the count of 1-2-3-4-5. If you want to increase the stretch sit upright with straight back. Now grab your foot and pull it upwards towards your face. When feeling comfortable you shouldn't need the assist of the thigh and raise the leg until it's hard on your chest and face with your hands. Repeat the exercise 20 times with each leg.
Assisted: Sit on the floor with a straight back towards a wall. Have the assistant push leg firmly until the whole leg and foot is hard on your chest and then hold for at least 30 seconds. After the assistand should slowly and firmly move the foot higher up and hold for 30 seconds. Do at least 20 repeats with each leg.
A very good yoga variation of this is the one shown above. It will also stretch your shoulders.
Sit on the floor and raise one leg as near to the picture above as you can and you can bend the back forward as needed.
To the count of 1-2-3-4-5 move the leg trying to get the leg behind the shoulder
later you will get the leg behind your back. Do the same with the other leg and repeat the exercise 10 times with each leg.
Assisted: You can have your assistant to help you get the leg behind the back as above in the exercise.
Like many of my exercises this starts sitting on the floor so that you have a good steady base. If you have this you do not get tense and feel afraid you will fall over. This allows better flexibility. Now raise one leg behind the head. Bend the back forward if needed. Put the other hand and leg in a comfortable position for balance. To the count of 1-2-3-4-5 push the leg firmly down the back as far as you possibly can towards towards what is shown above or even lower down the back. It does not matter if you have a bit of a bent back as this is natural but the hip socket is very strong so do make sure that the 1-2-3-4-5 pull down action is quite firm and that you are getting this foot pulled well down the back. Do the exercise 15 times with each leg. That foot is going to touch the floor behind your back sooner than you imagine!
Assisted: Have your assistant to firmly and slowly push the leg far down the back.
A teaser: One of my students is able to get the foot down lower than her waist, move around any way she wants and sitting and pulling it down she can, in time, put it on the floor so that the foot rests on the floor.
Here is another example of this...
...and one for the really advanced, a lotus behind the back. It requires that you master all leg exercises and dislocations of the hip joints.