Originally Posted by Bash and Crash
All of the above. Also in they are younger and smaller I will stand in front of them and put my hand up right where I want them to toss and have them toss up to my hand but have it barely touch my hand after a few attempts. You can also use a racket if they are taller. I also tell them to let their hand follow the ball up towards my hand to get nice follow through. If they are flicking with the wrist I also tell students to lift their arm from their elbow instead of lifting ball with hand helps to keep the arm extended and stops them from folding at the elbow joint. Have some more if those and TCF's don't help.
A similar technique is to have them stand under a basketball hoop in the correct stance/position to toss the ball up so it passes through the hoop on the way up and on the way down (works best with no net). In my own experience, getting youngsters to toss STRAIGHT up consistently is the hardest part. Once they can do that, you can tweak it from there.