As you sew your seam, GENTLY pull the sweater material both in front of and in back of the needle to stretch the fabric as you add the thread - let the feed dogs move the fabric thru, all you do is gently stretch the fabric evenly. This builds in stretch. Also use a medium size zigzag stitch...when the seam is finished, you won't notice that it's a zigzag and you'll have plenty of stretch in it. If the fabric looks a little weird when you're finished, wash the garment and the fibers will go back in place.