There are a couple of ways I know to increase flexibility for this kick, namely the hip socket and rotator, but they are kind of awkward. The position is strange, and even worse, most people are unfamiliar with the way the stretch feels, or how to relax the muscles to accomodate the stretch. The part of the socket in question is just to the inside edge of the femur, the thighbone.
To find the socket, sit in a chair, and track your thighbone up into your pelvis until you find the big crease caused by your flexed hip. When you hit that crease, push in a bit, and then trace your way inward and feel around. You'll find a mess of tendons. That's the area you're looking for.
Relaxing the hip socket is of utmost importance for these stretches.
One, you can lie down on the side opposite that of the hip you want to stretch. Bend the knee furthest from the floor 45