When you are sewing with a stretch fabric such as a knit or fabric with stretch fiber woven in, you may want to secure some of the seams so they do not stretch.  Shoulder seams and waistlines in pants or skirts are especially vulnerable to stretching.  Before you sew the seam, cut a piece of skinny woven ribbon or tape (straight grain, not bias) the length of the seam you want to sew.  Use the pattern piece as a guide.  Pin the ribbon over the seamline and sew through all layers.