For those here who believe that supination on throws has the potential for injury and that pronation is preferable, here is a drill that encourages pronation and discourages supination. I began each practice using this exercise regardless of age or level of play. The descriprion below addresses right handed throwers but you can adapt it to left handers.
Have two lines of players facing each other with a distance between them appropriate to playing level. Have the players kneel on the left knee with the knee at a right angle. No slouching. Have them place their right foot on the ground, again with the knee at a right angle and the toe towards his team mate. Have the throwing arm with the shoulder at a right angle and the elbow at the level of the shoulder and at a right angle. On the throw, insist that the right elbow passes on the inside of the right knee on the follow through.
Once they have it for the moment, have them stand and throw using the same follow through mechanics. Have them turn the instep of the right foot toward the person they are throwing to and step to him with the left foot. I prefer to having them use the crow step with the right foot when fielding a ground ball bringing it below the right shoulder and rotation of the instep to where the throw is going.