Ok I actually did make these for myself and they worked really well. I used 100% wool yarn. Rolled it into a ball and washed them in hot water. The only thing i would say is that if you use anything other than tights, for the washing, it is hard to get the ball out. For example, I used fishnets b/c that is all I had on hand and I had the hardest time separating the fishnet from the ball.
(via DIY Dryer Balls « greeneyed.com)