Basically, I tell new casters to start the casting stroke with a slow acceleration to a rapid stop. JC said it best when he said to start slow and end your cast like hitting a brick wall. Place your free hand out in front of your face, palm facing you with fingers extended upward. Start your forward cast slow, and end it fast when your casting hand HITS your free hand. It's the STOP, that propels your line. SLOW TO FAST & STOP, SLOW TO FAST & STOP.
I hope this helps.