Here's something from this site that may help.
http://www.flyanglersonline.com/begin/101/part10.php

I couldn't find the article but the late JC wrote one somewhere on this forum where he said hold the rod in your casting hand. With the other arm, hold your non-casting arm about an arms lenght in front of your face. On the forward cast, smak the rod into your outstretched hand so that it will make a sudden stop. That will get you to know the feel of what a sudden stop needs to be to get the line to go out there right.

I would advise practicing this way in the back yard instead of the lake as you may look a little whacky standing on the stream or at the lake slapping your rod in your hand. (or against your hand is more like it).

A ton of good stuff is here
http://www.flyanglersonline.com/begin/