My relationship with eka pada sirsasana is an on-going battle presenting challenges that I had never faced before. Blessed with a combination of a higher torso-to-leg ratio (long torso, shorter legs) and stubborn hips, asanas like eka pada sirsasana, dwi pada sirsasana, and supta kurmasana pose great difficulty for me (yoga nidrasana is slightly easier than these due to the help of gravity). I was inspired to do some more researching on the leg-behind the head postures this morning and found a few nice videos. My favorite one: Rob Lucas shows a unique way of stretching out the hips.
And of course, here's Kino McGregor, who makes everything look easier than it is...
And here's dwi pada sirsasana