I studied the book "Asthanga yoga - The intermediate series" by Gregor Maehle yesterday in the evening. I found some good tips for eka pada sirsasana. I still must hold my leg when it is behind my head. It simply won't stay there alone.
In case one is not able to do eka pada sirsasana Mr Maehle recommends two warm up exercises before doing the pose. One is seen on the picture: The shin bone is supposed to be parallel (not 90°). The other leg is stretched. The hips remain parallel and go down to the floor.
Do this pose and you will feel it. It works.