I found that standing up while machine quilting is much easier on my back. I created a styrofoam "table" to enclose my sewing machine so I have a large surface to work on, and place the machine and the styrofoam on top of my cutting table. (Instructions for creating the quilting table are on Youtube.)
For piecing, though, I still like to sit down. I tried tilting my sewing machine but did not find that it helped. It's important that the machine be low enough so that the flat bed is about even with your bent elbows. I use a pneumatic height-adjustable office chair with no arm rests and sit as high as possible to achieve that. Many years ago I used the weighted back thingie from Clotilde, and it did help. However, I have not found it necessary since working out the correct ergonomics for my current setup. I do get up and move around frequently while piecing; I never sew sitting down for more than 40 minutes or so at a time without getting up and moving.
I should add that my doctor sent me to a back and neck pain clinic several years ago. I would go in twice a week and exercise, under their supervision, using equipment specially designed to isolate specific muscles that support the spine. After completing that program, I have had no major back problems and only the occasional tiny twinge. This was after decades of intermittent back pain including episodes where I could not stand up straight to walk.