(Posted more than three years ago when I hadn’t tried ashtanga yet. Who would have thought that years later I would be working on backbends like anything!)
A tip on improving back bends: while in standing position, carry a 17-pound, squirmy baby in your arms and try to put him/her to sleep.
My niece won’t go to sleep unless I “dance” with her and she usually takes 20 minutes to doze off. By that time, my arms are so sore already that I need to bend backward to support her with my tummy and keep her from falling. I cannot put her down until she’s in REM state so I get to hold this position for a good 45 minutes.
I don’t think carrying weights or heavy materials (say, two Manduka mats) will work, as there won’t be any compelling reason to hold the pose for an extended period.