When I took classes at Stretch and Sew - the method was:
Use a longer stitch - stretch the fabric as one sews it - and then when the fabric is relaxed, there is some "stretch" in it.
Practice on some scraps. I think we used about 6-8 stitches to the inch when sewing sweatshirt fabric.
For added durability, sew a second seam about 1/16 inch away from the first seam - stretching the fabric again while sewing.