Excerpt from the sling article on
www.wikipedia.org (a free encyclopedia on the web that anyone can edit. It's really cool, you should check it out):
"The clumsiest part of using a shepherd's sling is to regain control of the released knot-end. Conventionally, the loop is placed around the strong hand's thumb, or held by the two weakest fingers of the strong hand. Several stones are held in the weak hand. After the release, an expert will continue the motion. The pocket will catch around a stone held out with the weak hand, so that the knot end swings back to the strong hand retaining the loop. Just after the knot begins to swing, slightly before the knot reaches the strong hand, one drops or throws the stone toward the ground with the weak hand, starting into the next release. Some persons braid the knot around a weight to help perform this maneuver. With this method, a skillful user can throw an aimed stone per second in a cyclic coordinated movement, until the weak hand is empty. "
For the complete article, see
http://en.wikipedia.org/wiki/Sling_(weapon). Really, you should check out Wikipedia. Anybody can contribute new articles, or edit the ones already there (they have over 500000). I'm sure you guys could improve the slinging article for example. The article states that overhand is the only practical technique, which we all know is incorrect. I suggest one of you guys who really know this stuff go there and update the article.