To get your leg up to the back, or, to do an arabesque, you need to get really flexible in the psoas muscle. This is a postural support muscle that runs from the top of your thigh to the spine. Attached at the front of the spine, this muscle controls the bending of the body at the hip joint. (Say you sneeze and one of your knees lifts up when your abdomen contracts. You bend at the hip joint).
If flexible, this muscle also allows your spine to bend at the waist (a spine extension), when you raise your leg to the back. Here is one of the best hip flexor stretches you can do, to stretch this large, important muscle.
For Non-Dancers an excellent video
For dancers and athletes - best hip flexor stretch:
You do not need ballet turnout to do this, so I am going to describe this exercise with your legs in parallel.
A Standing (Psoas Muscle) Hip Flexor Stretch
Stretch out one leg to the back, about a yard or meter behind you. Place the back foot flat on the floor. Hold onto a ballet barre or the back of a chair to support your balance.
Now straighten the back leg, and keep your spinal position as straight as you can. Your pelvis may tip back a bit. You will feel a pull up the front of the back leg thigh and hip. Pull up the lower abdomen muscles, and you will increase the stretch feeling.
Twist away from the back leg, a little. You will increase your flexibility this way, by getting into this position which is good for the hip stretches.
Twist toward the back leg, and again you will feel the stretch move somewhat in the hip joint area.
Hold each position for thirty to sixty seconds. Repeat the stretch at least three times, each position, each leg.
Always stop and decrease the position if you feel pain. Stretching means a stretchy feeling, a little tension, but not pain.
Get yourself a balanced lower body (with some dynamic upper body stretches) DVD flexibility workout including hip flexor stretching exercises.