Astavakrasana...Looks crazy, but is actually (IMO) one of the easier arm balances. Required? Hips + shoulders + abs...flexibility, strength, and strength respectively. Also though, you need to be able to use your adductor muscles like crazy. This bit is often overlooked.
You must be able to squeeze your legs together so strongly that you squeeze your arm tight. Here's a 35 minute sequence designed to get you to astavakrasana:
And just Eight Angle pose by itself...love Tiffany!
I love this pose. For all y'all crazyheads out there...try working this transition...I'm in the process of fine tuning...getting there on one side, the other has a long ways to go. Illustrated nicely via instagram JessiicaJade: