Shoulders that stretch out of shape are a frustration to many knitters. On cut and sew knit garments sewers are taught to use a piece of clear elastic which is sewn right into the seam. Normally it is pinned into place after being stretched out about 1/2 inch. It draws the shoulder up slightly without gathering it. The real advantage is when the sleeve is sewn in place. The elastic counteracts the tendency of the sleeves weight to pull the shoulder down. I've got two methods that work the same way for knitters. The first is to use your project yarn and a crochet hook. Measure the length of your shoulder seam, lets say it is 4.5 inches. Pin your shoulder seam down on a firm pillow and squeeze it up to 4 inches, then using your crochet hook, create a chain along the seam drawing it up the 1/2 inch. If you no longer have any of your project yarn you can use thread. Put a double strand through the needle so that you have four strands in total after knotting the end. Anchor it firmly and after using the same pillow trick to stabilize the seam, stitch along the edge of the seam making a knot every inch or so to compress the seam down. If it puckers on the outside you have gone to far, however depending on your gauge you should be able to easily remove 1/2 inch and often you can get up to 1 inch in total out of the seam length.