The chair makes a huge difference. I found that any chairs, even adjustable office chairs were just a little too low. Eventually I purchased an adjustable drafting chair ... it's lowest is the highest of an office chair ... and then goes up from there!
Works perfect at the machine ... and I can raise it if I want to work at my cutting table or ironing board (sometimes use it to work off, as it's oversized and just right!)
Classes so often have horrible chair/table combos. I take two chair pads with me, to gain the extra height ... and it lets me sit and sew so much more comfortably for the longer periods we push ourselves to, when in a class!
At home ... make yourself, take the breaks regularly!