I'm too tall so that I end up all hunched up and am in pain after a short time at the machine.
I have a sewing cabinet and my legs don't really fit under the cabinet. And I always end up with one foot on top of the other so I can use the pedal. Then I'm hunch over to see under the sewing machine arm.
I'm so glad you did this thread. The information is going to help me immensely.
How to change a bad habit is to make a sign where you'll see if when you are in the wrong posture. It will keep you aware so that after correcting the bad behavior constantly you'll find one day that your doing it right without thinking.