Alan Jaeger and Jim Vatcher are the best in the industry. We have been using their long-toss methods for over a decade and have had amazing results with our guys. It is the cornerstone of our training.
The key to the program is doing a great job finishing the long toss exercise. You want to emphasize pulling down on the release point as you move in gradually to 60'. The idea is to compress that 300' long toss into a 60' throw. When our kids learned to "finish" like that, their velocity immediately increased.