Shown below is a new sound etude that I've been practicing more recently that's been helping me build up my endurance as well as strengthening my altissimo register--which can be very hit or miss on the soprano.
I can barely play this etude more than twice in its entirety. I call it my corner-burning routine. It's patterned after the weightlifter's approach to working out, which is to isolate a particular muscle, tire it out with a repetitive exercise that enables you to tighten and release pressure to that muscle, which is to be repeated until the burning sensation becomes unbearable. And after you break the muscle down, it will become bigger and stronger, when it rebuilds itself. So that's the philosophy behind sound calisthenics.
Have fun with this etude. It works wonders!