Ironing works best, but the goal is to never actually touch the fabric: If you do, it can flatten and stretch the fibers permanently and leave an ugly shine. Get around this by turning the sweater inside-out and putting a damp cloth (white, so the color won't accidentally rub off on the item) between the garment and the iron. Gently press, lowering and lifting the iron instead of sliding it back and forth. Here's what settings you should use for best results:
Acrylic: Cool iron (use a dry cloth).
Cashmere: Steam, on medium heat.
Wool: Same as above, and then gently knead the sweater afterward.
Tip: You can use this same method to reshape shoulders that have been stretched out by a hanger. Turn the top on its side and lie it flat so the shoulder (and the offending peak) is facing straight up, and iron as above.
