The only effective way is to find a spouse or partner who actually cares about you.

(now single) Old fart here.

Otherwise this is my current one (they're all about 4/10 IMHO): Exfoliating Back Scrubber for... https://www.amazon.co.uk/dp/B0C1YZDFFM or something similar

As best I can.

My joints are over flexible, so i can reach everything just fine.

Water

~~I put liquid soap in the palm of my left hand. Turn one the water and put my right hand under, then let water roll off into my left hand. Turn off the water. Wash hands without wasting water. Turn it on to rinse.~~

~~There's a guy at my office who would let the water run for the entire five minutes that he brushed his teeth after lunch. I got so sick of it that I printed a sign and taped it to the wall. It said not to waste water and to turn it off while brushing. I haven't seen him doing it since, but I also haven't seen him at all since. I hope he saw it before someone took it down.~~

Edit: misread the title.

With my hands and a loofah. I'm double jointed in my shoulders and can easily reach every part of my back