The best advice I got when I was starting out with casting was to have a wrist motion like hammering. It isn't a semi-circular motion, but rather a linear motion with a snap at either end.

Another way to look at it: The guy at the fly shop had a short length of dowel with a punctured tennis ball on one end. Hold the dowel with the ball op in the air. Try to flick the ball off the rod with a circular motion - doesn't work.

Have fun.