My son (14U) worked this with a former Major Leaguer this past summer, and I wish he could continue to work with him throughout High School. Unfortunately, he's 3,000 miles away. During these sessions he worked almost completely on the attitude and thinking that is needed to be a Major Leaguer, fielding his position (i.e. controlling the decelerating and landing in a defensive position), taking responsibility for every pitch, and working on his backside decelerator. Never worked on increasing his velocity. I don't know how many times he said, "Throwing 105 means diddly if you can't get a strike." This coach kept him between 72 and 74 with control, confidence and command. BTW, on his 14th birthday, he was clocked at 89 mph (running start).
His doctor says he won't finish growing until early 20's. He's trying out for right field for his freshman year in High School. (He should make it. He has a gun for an arm and at 15 pound lighter than last summer, has good outfield speed.) He would also like to make the bullpen as a pitcher and continue to learn his craft, but is fine with not being a starter until his Jr year.