For a long time Urdhva Dhanurasana, or full backbend, was a real challenge for me. It takes everything and it really takes it out of you. Exhausting.
Rather than write anything, the best possible source of information for these poses is Christina Sell. I still think of things I learned in her classes every single time I go up into a backbend.
For Dwi Pada Viparita Dandasana, this trick of hers changed everything for me
Unfortunately I've been sidelined on these postures for the past couple weeks due to a pulled muscle in my chest due to coughing (I know right?!?!). It's finally almost gone, so hopefully I'll be wheeling before too long.