This post is kind of out of date. The night before the craft fair I decided I'd attempt to kill the acrylic headbands to help them lay flat. Basically I pinned them to my ironing board (which has 2 layers of padding on top) and filled my steam iron to the tippy top with water. I then went to town pressing my steam button allowing the steam to hit the yarn without allowing the iron itself to touch it. It worked like a charm!
Killing yarn alters some of the fibers in the acrylic. It reduces it's elasticity allowing it to drape more fluidly. It also sometimes gives the yarn a nice sheen.