Yes, good ergonomics are KEY to comfortable sewing!!
Another thing I haven't seen mentioned yet is to try a wedge cushion on your chair. It will lift you up a little and (for me and my particular chair, anyway) I find that if I'm tilted just ever so slightly forward it encourages me to have better posture in my lower and mid back. If this gets your feet too far off the ground, put a box or buy a foot rest and make sure you have good foot support as well.
A last idea - to help break bad habits, set up a mirror so you can glance over at yourself periodically while you're sewing to check your posture. I have a terrible habit of scrunching my shoulders up almost to my ears while I'm quilting a quilt sandwich; a glance in the mirror helps remind me to keep my shoulders DOWN. I LOOK uncomfortable when my shoulders are up, and it doesn't feel good either. I do it out of stress...for me the quilting phase is the most stressful and I "clench" myself all up while I do it, which is bad for the quilt and really bad for my neck and shoulders! Learning to relax is hard for me, but I'm working on it and feeling better and better after every quilt.
Maybe you're a "clencher" too?