Just lots of patience to get from relaxed throws to long toss to walk-thrus and finally full motion. Oh, and of course about 20k reminders along the way .
It always amazes me which cues work for different kids. Sometimes, just telling them to feel a whip or snap at the pocket does the trick. Others need the IR progression. "Eyes closed and feel the whip" can be helpful. Some kids do better when they try a full circle. Reminding them to pull the loose, lower arm around into the whip can help. Let the lower arm, hand and ball just hang from the elbow. Remind them not to aim and that whip initially feels out of control. Let it whip--don't throw it.
Posture is always a part of the progression and most times, I encourage kids to step or walk into the whip. Always roll up onto the toe and gently squeeze the knees. I demo palm up at 9 o'clock and sometimes have them look back at the ball. Sometimes I use a ball with a dot marked between the narrow seams and ask them to throw the dot (pinky-side) at the net/target.
An adult catcher or net, with no emphasis on control is a key. I don't care where it goes as long as it looks good.
Fastpitch Dad's suggestion of feedback is critical. Slow mo video is also very helpful for both coach and kid. As they begin to show a little consistency, ask them if a whip was good or not. As soon as they're able, I want them to tell me "good" or "bad" before I can say something AND why.
Write a letter, troll grout, shovel concrete, brush paint a wall, throw a frisbee, hit a ping pong ball...
Bill has some video's out on YouTube that you may wish to take a look through. He describes his view about the importance of working with the hands together, as well as the role of the hands staying above the waist can play in helping to promote what you're interested in achieving.
An example of what that would look like...
Note, the throwing hinge (elbow) isn't allowed to achieve a lockout position at any point in the arm circle, from 6-to-6. The elbow is thus allowed to work it's hinge at all points inbetween which in theory will allow for elbow whip and "I/R" to occur naturally.