Most of us sew on a dining table, while some have the luxury of setting the machines in a sewing cabinet that allows you to have the sewing machine bed level to the table. Regardless of whether you have a specific table or not, it's important to note that the bed of your sewing machine, where the stitching takes place, needs to be parallel to the bottom of your ribcage. It automatically sets your back straight, forming a 90-degree angle with your arms over the sewing machine bed.

Depending on your body type, you may need to adjust the height of your chair to achieve this. I have my machine on a regular desk at home, which means there's an additional 4" height to reach my sewing machine stitching area. Sitting on a standard-height chair is too low for my arms to form a 90-degree angle - my hands are raised, wrists bent, back slouched, and shoulders are up. Doesn't sound too comfortable, does it?
I would stack some books and pillows on my seat to sit at a comfortable height. Eventually, I purchased a bar stool with a backrest and found it was the right seat height for my body type to establish a strain-free sitting position.
Of course, a height-adjustable chair will do well. The images shown here are of me at the store, by the Koala sewing cabinets - they have hydraulic-powered surfaces that allow you to set the height of your sewing machine.